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How to test and troubleshoot gas furnace pressure switches.


Question: How can I test to see if a pressure switch is operating properly?


  • Set your meter to volts AC, turn your furnace on so it is calling for heat. The draft inducer should start.
  • There should be either two or three wires going into your pressure switch.
  • Probe one lead of the pressure with one lead of your meter while touching the other lead of your meter to ground. The ground would be any bare metal part of your furnace. This must be bare metal. I always try to touch the other lead of my meter to the furnace’s switch box.
  • If you have a two-wire pressure switch you should be getting 24 or more volts between both leads to ground. By this I mean if you touch one terminal of the pressure switch with one lead of your meter, and touch the other lead of your meter to a ground, the bare metal part of your furnace, you should get 24 or more volts (24 to 28 volts).
  • If you do not get 24 or more volts with the furnace running then you have a pressure switch problem.
  • A gas furnace pressure switch problem could be caused by the following:
    • Your vent could be stopped up with a bird’s nest, wasp nest or debris.
    • You could have a bad, dragging, dirty or slow running draft inducer.
    • The tube that runs from your pressure switch to the draft inducer could be plugged up. Many times the tube is plugged up with water. If the pressure switch tube has water in it then this is a sign that your condensate drain line could be stopped up or not draining properly. You can disconnect the pressure switch tube and run a wet vac to hopefully clear the stopped up furnace condensate drain line.
    • The draft inducer hole could be plugged up. I have seen the draft inducer holes get stopped up so bad that I had to use a small drill bit and drill to open the hole back up.
    • On high-efficiency condensing furnaces, the water drain line could be stopped up, causing a water back up and blockage in the pressure switch tube. *Make sure you disconnect the pressure switch tube before using a wet vac because the high vacuum of the wet vac can damage the pressure switch. I usually take the drain hose loose from the condensing furnace and use a wet vacuum to open the drain back up.
    • I wanted to post this because many people are telling me that they are sucking on the pressure switch tubing to test the pressure switch operation. I have heard from other forums that you should not suck on the pressure switch to test it because this can damage the diaphragm inside the pressure switch. It would be best to use a manometer or test with a voltmeter like we advise on this page. I wanted to pass this along so others will not damage pressure switches.
  • Please click here if you are interested in seeing the furnace pressure switches that we sell
  • Please send us your furnace’s model number if you would like for us to recommend a pressure switch or part for your furnace. Our email address is:
  • Below we have a real good Youtube video made by GrayFurnaceman that shows how to troubleshoot a pressure switch on an 80% furnace. Thanks to GrayFurnaceman for making this informative video! I hope this will help you troubleshoot your gas furnace pressure switch.

85 thoughts on “How to test and troubleshoot gas furnace pressure switches.

  1. Hi Steve,

    I have a Goodman GKS91155DXAF. Few days ago, it did not turn on. I removed the cover and when I pressed the limit switch (under the cover), the inducer motor turned on then the igniter turned on but after that, the inducer motor turned off. I released the switch then pressed the switch again and this time the furnace went back to working properly. Next day, same problem and fixed the same way again. Today (three days later), it stopped again.
    When it is working, I hear the two clicks. When it doesn’t, I hear one click only. Would this be the pressure switch or gas valve?


    1. Hi! This sounds like you have a limit switch problem since the furnace started working right when you pressed on the limit switch. If it is the pressure switch you are pressing on to make the furnace work then the pressure switch part number from our Goodman parts program is part number: 0130F00000P The manual reset limit is part number: 10123519 The gas valve is part number: 0151M00013 I am sorry we do not stock any of these parts. I would like to suggest that you check with a volt meter to make sure that the pressure switch and limit switch are staying closed when the draft inducer is on and the thermostat is calling for heat. I hope you can easily find and fix the problem. Steve

  2. It should also be noted that furnaces can trigger a pressure switch fault even if the switch is not bad.
    I replaced both the pressure switch and the inducer fan (because they were old anyway) but this didn’t solve the issue. The reason the pressure switch was faulted was because the inducer did not run to activate the switch. Original Inducer fan was old and rusty but it wasn’t out. Turns out the furnace board was bad (probably bad relay) so it wasn’t sending the 120 volts to the inducer to make it turn on. You can also test this by directly running 120 volts to the fan manually from an outlet. I have been able to temporarily run my furnace manually like this until the new board arrives. But, I definitely don’t recommend this unless you are monitoring it when running and for short a short duration (like 30 minutes or something).

    1. Thank you so very much for telling us about your experience and solution to this problem! I really appreciate you taking your time to explain and post this! God bless you and your family today and always! Steve

    2. Thanks for taking the time to sharing this. I am still getting the closed pressure switch error. The thing is, when I switch the power off and turn it back on, the fault seems to cleared and the furnace runs for some time (the time varies from one instance to the other) and then shuts off with the closed pressure switch. There are also instances when it would run just fine without a fault. May be it is worthwhile just swapping out the pressure switch and the board, like you said.

      Many thanks again for sharing your experience.


      1. Hi! This sounds like the pressure switch is getting stuck closed and not opening up after the draft inducer shuts off. If you have a condensing furnace that produces water please make sure that the condensate drain line is not stopped up. A water build-up will cause the pressure switch to stay closed. I hope you get it fixed soon. Steve

      2. Hi Steve,

        Thanks again. Our furnace (GPD125-4) does not appear to be a condensing furnace. Do you have the pressure switch and the control board in stock? I am wondering if I should replace them?

        The pressure switch reads:
        B13701-26 [L]
        D/C 31/99/CR

        The present control board reads:
        B18099-13 1012-933C.


      3. Hi Raj! I answered you through your email address and ours I sent you some attachments for your furnace’s parts list and service instructions. I sent you links on our site where you can purchase the parts. I hope this helps you find and fix the problem with your furnace. I hope you have a great day! Steve

  3. Hello Steve,

    Would very much appreciate your thoughts on this.

    We have a Goodman GPD125-4 furnace. It works for some time, then I find that it trips with the 4 blink signal (Open limit switch) first — I can hear the blower fan running for some time, which makes me think the furnace got overheated. After some time, I can hear the blower fan stop and I notice the 4 blink signal change to 2 blinks (Closed Pressure Switch). I think by this time the furnace has cooled down.

    I replaced my air filter and that didn’t fix the issue.

    I do find that when I switch off and restart the thermostat on the wall, the furnace starts again but only to stop after some time.

    I believe the furnace was installed in 2000. Is there a timeline guidance for these types of furnaces?

    Also, I couldn’t remove the hose around the pressure switch and the draft inducer — on both sides, it seems to be “glued”. And, lastly, once I take the cover off the furnace, I find that the housing around the limit switch is fairly hot to touch. Is this normal? Should I read the temperature on it with a TC?

    Thank you very much, in advance, for your time and suggestions.

    1. Hi Raj! If you have air conditioning installed on your furnace you might have a partially stopped up evaporator coil. When I was doing service work many times when furnaces were going off on limit I would inspect the underside of the evaporator coil and find the coil to be stopped up with lint and dirt. The following is what I have listed on many of the pages where we sell limit switches: FYI: A furnace limit is a safety device that is used to cut the gas burners off if the furnace over-heats. If your furnace is going off on limit you might have an airflow problem like a dirty filter, dirty blower, slow blower motor (might need a new capacitor) stopped up evaporator coil or too much gas pressure. All of these can cause the furnace to overheat and go off on limit. Many times when the furnace goes off on limit the fan blower will continuously blow because the furnace control board senses that the furnace has overheated and needs to cool down. The only way to get the blower to cut off is to turn off the furnace with the power switch. You can test a limit with a voltmeter to see if it is open or not. On most limits, you should get 24 volts from each of the two wires on the limit to a good ground. I would suggest checking to make sure your furnace’s blower motor capacitor is in good condition and up to the specs that are printed on the capacitor. If the capacitor is weak the blower will run slow and cause overheating. You might consider moving the blower motor speed up to a higher speed in the heating mode on the control board. Make sure you return registers/grills and supply registers are all open and unobstructed. You might need to adjust the gas pressure down to lower the btu gas input rate. Yes, It is normal for the limit to be too hot to touch because some of them go up to temperatures above 160 degrees and anything above 120 degrees is considered scalding with water temperature. So, yes the furnaces do get hot, but your furnace should not go off on limit unless there is a problem. I hope you can find and fix the problem soon! Steve

      1. Hello Steve,

        Thank you so much for this detailed response. Much appreciate your time and timely help.

        A very happy and warm thanksgiving to you and to your dear ones.


      2. Thanks so very much for your kind words and Thanksgiving well wishes! The same to you and your loved ones! Steve

  4. Hello Steve,
    Your website and comments was the key piece of information I needed to resolve my furnace problem. I have a Lennox Elite Series that would shut down after running for a few minutes. I could reset it and it would come back on, but then shut down again. So I did the following;
    1. Replaced the flame sensor – it didn’t fix the problem (and it is very difficult to access on this unit).
    2. Checked the limit switch – all OK.
    After these checks the unit started to just turn on and shut off in about 2 s. It kept trying to relight and would light, but immediately shut down. The change from running for a few minutes to running for 2 s was just coincidental, but it was clear that there was a sensor shutting the gas down. I then;
    3. Checked the resistance on the two vacuum sensors on the firebox blower motor. There are two of them on this unit. To do this I turned off the gas valve and let it run. The top vacuum sensor had zero resistance so was a completed circuit. The bottom one was zero, then would jump to infinite, then come back down to zero, so I suspected the sensor was faulty. I then turned the gas back on and sure enough after lighting the burners, the sensor spiked to infinity and the gas shut off at the same time. After ordering a replacement part I was looking at some more websites on the vacuum (flow) sensors and found your website. You made a comment that one should check if there is water in the vacuum lines from the blower motor to the sensors and the light bulb in my head went off that the intermittent failure on the flow sensor I was seeing was probably water in the line. Sure enough, I pulled both vacuum lines and a bit of water came out of the red one on this unit. I made sure the hose was clear, replaced the vacuum hoses, and tested the unit and it ran fine.

    In the past I have had intermittent shutdowns, but usually I could just shut off the unit at the thermostat and restart it and it would run normal. Now I know to check for water in these vacuum lines first if this happens again (it’s a much easier check than the flame sensor).

    Thank you for being generous with your time to help do-it-yourselfers. I live in a remote location and it is very difficult (and therefore very expensive) to even get someone to show up. It will be cold next week but warm inside now!

    Best Regards,
    Hank Oviatt

    1. Hi Hank! Thank you so very much for your kind words and for taking your time to explain how you solved your furnace problem! I am so grateful to you for taking the time to help others who might experience the same problem you did with your furnace! You are a very kind and generous person for writing this to help others. God bless you and your family today and always! Many Thanks to You! Steve

  5. Hello,
    I have a ruud achiever 90+ furnace.
    The inducer went out on it, up until then it worked fine. I replaced the inducer, and now the inducer turns on properly. However, when the inducer turns on, it does not pull enough vacuum to pop the pressure switch. When I take the inducer off and cover the exit hole partially, or the air entrance hole most of the way it pulls enough pressure to open the pressure switch. I have made a seal around the inducer.

    I replaced this inducer with the exact same part number, rpm, amperage, etc.

    I did have moisture issues in my furnace last year as my drain was plugged. I dont know if this would cause corrosion or sticking inside of the pressure switch.

    Any thoughts?

    1. Hi Jacob! Since you say that the pressure switch gets enough vacuum when you cover the exit hole partially or air entrance hole then I would guess that you have a leak in your seal combustion furnace somewhere. On most 90% furnaces, they are sealed combustion and if you have a leak anywhere in the combustion chamber then the pressure switch will not close. What I am trying to say is there might be a leak that is allowing extra air into the combustion chamber and the extra incoming air does not allow the draft inducer to produce enough negative pressure to close the pressure switch. Just a guess. I would suggest that you make sure that you do not have any leaks in the burner cover ignitor wires or anywhere there is an opening into the combustion chamber. I have a Bryant 90 plus furnace and I changed the ignitor out and in doing so I bent the sealed ignitor bracket enough to cause a leak in the combustion chamber right around the ignitor bracket. I could hear air being sucked in around the ignitor bracket. My furnace would not work until I installed a new leak-free ignitor bracket. So make sure everything is sealed. If everything looks sealed and you are still having problems then I would suggest replacing the pressure switch. If you want me to look up the pressure switch for your furnace then please send me your furnace’s model number and serial number to our email address. I hope you can find and fix the problem soon! Steve

  6. Hi Steve. Im a bit at a loss here. Ive owned this house for 10 years and every so often I have had to repair my furance, and this is the only time I’m beyond lost as whats causing the issue. I have a carrier weathermaker 9200 model # 58MXA060-12. I am receiving error code 31, which states : pressure, draft safeguard, aux-limit switch or vent switch is blocked or closed, low inducer volatage, defective blower motor or capacitor, defective pressure switch, inadequate combustion supply, defective inducer motor, restricted vent, excessive wind etc. In my diagnoses and repair, I have completely removed the condensation trap, washed it out, reinstall and it flows perfectly, I get same error if I remove the pvc intake vent, I have fully replaced the pressure sensor, and fully removed the inducer motor component, fully removed the silicone on the backside of the triangular piece and also within the inducer motor component and replaced with new silicone, Ive blown and washed out all the tubes that connect the inducer motor to its secondary heat exchange, I have tested the voltage on the pressure sensor and I get 26v now after cleaning the thermostat probes and replacing the batteries, prior i had around 23.5 volts, I have setup a menards $10 manometer and we receive around 1.1 water column on the bottom tube of the pressure sensor which seems too low for what this pressure sensor wants. If I remove the wires from pressure sensor and wait til inducer starts up and than jump the wires for the pressure sensor the furnace works perfectly. If I was a carbon monixide tester around all vents and the furnace I receive 0 ppm, on the exhaust pvc pip that goes outside I receive around 10ppm on the reading. I know this issue revolves around some sort of pressure issue and with re-silconing the inducer mount and inducer fan module and making sure all tubes are fine from pressure sensor to condensation trap im at a lost why im still not receiving enough pressure. Even disconnecting the intake gives same error code 31. Im skeptical on removing the exhaust pvc to retest as the smell becomes strong once inducer motor comes on to verify if the exhaust pvc pip is semi clogged. Is there a way to fully test if the control board is fully functional or is there any other recommendations you might have that I could investigate that is causing this. I know this unit is under the lawsuit of failing 2nd exchanges, but removing the blower motor and looking up I see 0 signs of rust or other issues that is the issue.

    1. Hi Paul! So very sorry that you are having this problem with your furnace and it looks like you have done just about everything possible to try and correct the furnace problem. One question that I would ask would be is the pressure switch actually opening up and breaking the circuit during a heating cycle? I assume that the pressure switch is opening up, or will not close to start your furnace sequence of operation. Since you replaced the pressure switch with a new one then it almost has to be some restriction in your venting system. The only thing that I can think of that might be the problem would be a stopped up secondary heat exchanger. As I am sure that you know the secondary heat exchanger is like a radiator and can become plugged up with debris rust and water. When you said that the smell becomes strong after the draft inducer starts with the vent removed is a sign that you have problems within the heat exchanger. I remember that I have seen this before and it would give an unburned natural gas smell that was not a good smell. I would be interested in seeing how your gas burners are burning when you by-pass the pressure switch. If the gas burners start off burning properly and then after 30 seconds or so start to burn back (flame gets shorter) then if the burner almost goes out altogether then this is a sure sign that your heat exchanger is stopped up. From reading your post it sounds like you already did this, but if you can disconnect the pressure switch and try to use a wet vac to suck and clean out the secondary heat exchanger then this is what I would suggest that you do. I am sorry, but it sounds like you have covered all other areas of troubleshooting the problem. You might have to either get a new secondary heat exchanger or a new furnace. Sorry that I can not be more help. I sure hope you can find and fix the problem soon. Steve

      1. Well the new pressure switch is part # HK06NB124 which I guess replaced my original one ending in 0. I can only find one place that lists the rating of this unit which only gives me max psi .07 which is about 1.94 wc. I cant find the minimum rating so that it actually opens up, and when I tested it was only about 1.1wc.

        Now previous I was able to do a com24/test to test the motor and ignitor. My next plan was to run that test and recheck the wc while the blower is on high speed, but maybe im not smart or know how to properly check the vac of the pressure sensor. As only the yellow cord, one that is lower on the unit gives a 26v when the heat register turns on, since the unit gives me error code 31, i cant ever get a reading on the orange wire, at the top of the pressure switch. Now if i jump them obviously thats giving me one reading for both wires so thats not 100% accurate measuring. So with the com24/test not working anymore it makes me think that control board is having a short somewhere?? Not sure what I need to do to get that test to work again. In response, the burner flames seem to be the exact same size upon startup and at least 2+ min later on, they do not change in size. But I know there is some other issue because I cant ever get this house all the way up to temp. Thermostat is at 74, and lets say house is at 65 and i jump the pressure switch and leave the furnace on, it never seems to get above 70/71 before the error code 23 kicks on ( meaning i have the pressure switch jumped before inducer comes on) so im not sure why the inducer is deciding to stop running before it reaches its set temp.

      2. Hi Paul! So sorry to hear that you are still having problems. About the only thing that I can guess would be that the secondary heat exchanger is partially stopped up and you probably either need a new secondary heat exchanger or a new furnace. We sell the HK06NB124 and our pressure switch we sell does not have the amp rating or water column rating on it either. I have installed several of the HK06NB124’s on furnaces that needed pressure switches and the new pressure switch fixed the problem I hope that you remembered to disconnect the pressure switch tubing before using the wet vac. If you forgot you probably ruined the new pressure switch. If the furnace stops before reaching the set temperature I would suggest checking the limit to make sure it is staying closed and make sure you are getting a constant 24 to 28 volts AC between the W and C thermostat wires. Something is cutting the burners off and you will need to find it. It could be a control board problem but if the control board is showing a flash code pressure switch problem then more than likely it is not the control board. If you would like for me to find out which control board or pressure switch your furnace uses then please send me your furnace’s product number to our email address: and I will look up the parts and show you the pictures of the part numbers. I hope you can find a solution to the problem soon! Steve

      3. and to clarify if the thermostat does reach temp it will turn off the inducer, but sometimes it turns off beforehand. also 15 min later and the burner flames are still the same size after bypassing pressure switch to warm up the house. I did take out the blower fan and crawled inside and looked up and saw 0 signs of brown or white residues. I also did use a vaccuum to suck from the bottom grate down and also whatever i could reach within the black triangular piece behind the inducer when I had that removed.

      4. This sounds like you have tried just about everything to find and fix the problem. Sorry that I can not give you any more advice to try and find the problem. Steve

  7. Hi Steve! I have a Rheem Classic package unit. The furnace will light and run for about a minute and then go off. I have tested the flame sensor and I get 3 amps when hot. I’ve tested A/C voltage from the pressure switch and I’m getting around 23.5 volts from each when running. Where should I look now? Thank you sir.

    1. Hi Tracy! I would like to see above 24 volts on the pressure switch. You might have a weak transformer or another component like a gas valve or shorted thermostat wire that is making the voltage low. Most of the time the voltage is between 24 and 28 volts AC. I would suggest making sure the batteries (if you have batteries in your thermostat) make sure they are good. Make sure you are getting a constant 24 to 28 volts between the W and C (com) thermostat wire when your thermostat is calling for heat. If you are not getting a constant 24 to 28 volts between W and C then you might have a thermostat or thermostat wire problem. I would check the voltage between R and C and if you are not getting a minimum of 24 volts there then I would suggest that you check all wire connections to make sure they are good and tight. If all wires are good and tight then you might want to try and get a new higher VA transformer. I would also suggest testing to make sure that your limit is staying closed. If the limit opens up, which shows an over-heating condition then the gas burners will go off. I would also test the gas valve to see if you are getting 24 volts to the gas valve. If you are not getting 24 volts to the gas valve then you might have a control board problem or if you are getting 24 volts to the gas valve and it is not on then you might need a new gas valve. I hope you can easily find and fix the problem. Steve

  8. Hi – My furnace works just fine most of the time, but occasionally it shuts down just after it starts up. It will run fine all day and then randomly (usually in the morning for some reason) it shuts down after just a few seconds. This is before the flames start. I get the “pressure switch open with inducer on” error light. Filter is new, I cleaned a bunch of dust off the air vent on the blower motor the other night and it seemed to work fine for a couple days, and now it has started happening again. The tubes in and out of the pressure sensor look clean, I’ve pulled them off and where they plug in/out of are not clogged. I’m wondering if the switch itself may be getting flaky and needs replaced. I don’t think there’s a vent clog or else it wouldn’t work as well as it does most of the time, right? Thoughts? Thanks!

    1. Hi Jack! Since your control board flash code is saying that the pressure switch is open, then I would recommend trying a new pressure switch. The other problem with the pressure switch staying closed would be a partially stopped up vent (maybe you are getting a downdraft when the air is cold in the morning?) If your furnace has a condensate drain then make sure the condensate drain is open by removing the pressure switch tube and cleaning the drain out with a wet vac. If you leave the pressure switch tube hooked up then it will damage the pressure switch from all the negative pressure from the wet vac. A slow draft inducer or a draft inducer that is leaking air can also cause this problem. Make sure the draft inducer is installed tight on the furnace body with no leaks to the outside air. Most of the time lockouts that occur in the morning when the furnace is cold occur because the flame sensor is dirty or needs replacing. Flames with start and go out within a few seconds. I would suggest that you clean the flame sensor and make sure all wire connections are tight to make sure you have covered all possibilities. I hope you can easily find and fix the problem. Steve

      1. Thanks! I’ve done/checked all the other stuff you mentioned, and like I said, since it runs just fine like 99% of the time I think all the other possible issues are not the cause in this case. I’m betting on a worn out or sticky pressure switch.

      2. Yes, sounds like it could be the pressure switch sticking ever once in a while. If you would like for me to try and look up parts please send your furnace’s model number to our email address I hope you have a great day! Steve

  9. I have a lennox G26. The burners were not staying lit so I took the cover off to clean the heat sensor. To get to the sensor I had to take off a burner box cover. After I cleaned the sensor the furnace worked, but as soon as I put the burner box cover back on it it began cycling again and burners would not stay lit. Two questions: 1) can I just leave this burner box cover off tonight? 2) is this likely a pressure switch issue?

    1. Hi Van! I do not recommend leaving the cover off because this could cause more problems with your furnace. I would suggest that you try to fix what is causing the furnace to stop when you install the burner box cover. I would like to suggest that you check the vent to make sure the intake and exhaust are open and not obstructed. I would suggest taking the pressure switch tubing off so that the pressure switch does not get damaged and cleaning the condensate drain and draft inducer openings with a wet vac. A stopped up condensate drain and heat exchanger can cause the pressure switch to open up as you described. Make sure that the draft inducer is clean and if the draft inducer has a capacitor that the capacitor is up to specifications. If you need me to look up parts I will be glad to try and do so. Please send me your furnace’s model number to our email address at I hope you can easily find and fix the problem. Steve

  10. I have s similar issue, with a Goodman 80+. I set the temperature back at night to 65 degrees and in the morning has a call for heat, the inducer motor starts, the igniter glows and the furnace fires.. The unit runs for about 10 to 15 minutes (does not reach the set temperature 72 degrees) then the inducer shuts down, main blower continues to run. After about 60 to 90 seconds the inducer runs and the furnace lights again. I’m at a loss, the furnace tech has replaced the board, both high temperature limit switches and the flame sensor. The furnace was installed in 2006 and developed this issues about 3 years ago.

    1. Hi Bill! This sounds like the furnace is going off on limit. I am guessing that after 10 to 15 minutes of run time the heat builds up enough in the furnace to cut the furnace off on limit. I would like to suggest that the next time the furnace cuts off that you test the limit to make sure that the limit is closed and not open. If the limit is open then you have an overheating condition that could be caused by any one of several things, like a dirty filter, filter that is too thick and not letting enough air through the furnace, slow blower motor (motor might need a new capacitor or change to a higher blower speed), dirty blower wheel, partially stopped or dirty underside of the evaporator coil (coil might need cleaning), obstructions in the ductwork (make sure all registers and dampers are open)or the gas input on the furnace might be set too high. You or your service tech might need to adjust the gas valve so the furnace does not have so much gas input and produce so much heat. If there is too much gas input and not the manufacturer’s recommended heat gain from the ductwork (measure temp) to the top of the furnace (measure temp) then this will cause your furnace to overheat. I hope that this helps you solve the problem. Steve

      1. Thanks for the information Steve, I will schedule another service call and keep you posted.

      2. I hope you can get it fixed inexpensively. Yes, Please let me know what the problem is. Thanks! Steve

  11. Love love this video! You hit the nail on the head. Air inducer won’t start until 1 lead is disconnected from pressure switch! So, switch is stuck closed. That means I need a new pressure switch, correct?

    1. Hi Sue! Yes, you probably need a new pressure switch. I would suggest making sure that the pressure switch tube is not stopped up and make sure the draft inducer hole where the pressure switch tubing is attached is not stopped up. If the pressure can not release the pressure in the tube, then the pressure switch will stay closed. Make sure that you do not do suck on the pressure switch tubing, use a wet vac or subject the pressure switch to any pressure because it can ruin the pressure switch. If you would like for me to try and find out which pressure switch your furnace uses then please email me with your furnace’s brand name and model number. Our email address is I hope you can get the furnace fixed soon! Steve

      1. Thanks for the quick reply Steve. Once I take a wire off the pressure switch, the air inducer starts. I put the wire back on the pressure switch & the unit runs until it reaches set thermastat degree, then shuts off like normal. I am having trouble getting the short sturdy orange tubing off, to check for blockages. They are on good. I think I will have to take pressure switch off to get tubing off. So, I will be sure to check for blockages when I remove old pressure switch. But as it seems to function normally, once I get inducer to start, I’m thinking there are no blockages. I have Lennox Honeywell part # & already found the nearest Lennox dealer to me about 18 mi from home, my son actually used to work for them. Price is $38. Honeywell part #: 49L9001. If you can get it for less, including delivery, I’ll gladly get it from you. Thanks again for your wonderful video & quick response! 🙂

      2. Hi Sue! Glad to hear that you found that the pressure switch is the problem. Yes, sounds like the pressure switch is sticking closed because when you take the wire off one of the terminals on the pressure switch this opens the circuit and allows the furnace to start the heating cycle. Unfortunately, as you know you have to take the wire off for every heating cycle which is not good. I am sorry we do not stock the pressure switch that you are looking for. I am glad that you found the pressure switch somewhat close to your home. Thank you so very much for your kind words and correspondence. God bless you and your family today and always! Steve

  12. Damn are you helpful Steve.

    I have a Lennox high efficiency furnace in my home that I think I have a pressure switch issue. The draft inducer turns on but furnace does not fire up and will repeatedly run the inducer of and on. If I take off the door and pull off the orangish red I think it’s silicone tube (I think it’s the negative) and then place it back on it will fire up no problem then when it reaches thermostat temp it’s a crapshoot whether it fires up the next time thermostat triggers heat. Thing is it’s done this the last couple years and it’s alwAys started early spring/ late fall but usually after removing the tube mentioned above once or twice it works like 100% normal until the next seasonal change. This year it doesn’t seem to last longer then a day ( sucks waking up in a 15C house hold). I tested the furnace here or there in the summer and it fired up no problem.

    I purchased what I thought was a replacement switch from Lennox but it looks different than the one installed, is that normal or was I given the wrong switch?

    1. Hi Jeff! I would like to suggest that you look at the WC (water column) rating on the two pressure switches and compare them to see if you received the right pressure switch. If you want me to look it up in our Lennox parts program then please send me your furnace’s model number and I will see what the part number that Lennox recommends. Please correspond with me through our email address: so I can send pictures etc which is not allowed in this forum. I would like to suggest that you take the pressure switch tubing off the pressure switch and take a wet vac and make sure that your condensate line and everything is clean. Do not leave the pressure switch hooked up while using the wet vac because it will ruin the pressure switch with too much negative pressure. Best of luck! I hope you can get it fixed! Steve

      1. Thanks for the quick reply. They both say 0.65 on them so I believe they gave me the correct switch just from different manufacturer.

        Which line is condensate line?

      2. Hi Jeff! If you have a 90% condensing furnace there will be a condensate line that drains the furnace. If you do not have a 90% furnace then you will not have a condensate line. The condensate line is usually a plastic or rubber tube that drains water off the heat exchanger. If this is stopped up the pressure switch will not work. Best of luck. Steve

  13. Steve, have a Tempstar ntg3100gfg1, would fire, but quit after 5-8 seconds, cleaned flame sensor, no difference, changed control module (S8600H instead of S8600M) now fires for about 12-15 seconds, what is my most logical next step/replacement?

    Thank you in advance, in Michigan, kind of cold out.

    1. Hi Hugh!
      This sounds like a flame sensing problem. Please make sure that all wires, especially the ground wires are good and tight. Make sure the control board is tightened down with a grounding screw. Our Tempstar parts program shows control ignition module part number 1011449 which is a Honeywell S8600. I am sorry, but I do not know the difference in the “H” and the “M” module. That could be your problem. We have a good post on troubleshooting flame sensor problems on the following page: There is a really good Video near the bottom of the page made by AC Service Tech. I hope this helps you out. Steve

  14. Greetings!

    Our furnace is a Goodman GPD125-4. I have seen the furnace shut down recently without reaching setpoint and upon opening its panel found the Red LED blinking twice indicating a pressure switch close issue. I tried to see if I could clean the pressure switch tubing but it seems pretty hard to come off either from the pressure switch or from the inducer motor assembly. I think our furnace is about 18 old. Not sure if this tube or pressure switch was ever changed. Do you have any recommendations on how one might go about removing and cleaning this tube? Thanks!

    1. Hi RD! More than likely the pressure switch tubing has gotten hot and melted on the metal part of the draft inducer. More than likely if you have enough slack in the tubing you will need to take a utility knife and cut the tubing off then cut the melted part off the draft inducer connection port. You might want to take a piece of the tubing to your local auto parts store or Home Depot and get one to match. I looked it up in our Goodman program and your furnace uses pressure switch part number B1370126 which we sell on the following page: I copied a parts list for your furnace below. I hope you can easily find and fix the problem. Yes, I would check to make sure the opening in the draft inducer was not restricted or replace the pressure switch. Happy New Year! Steve

      Total: 45 Model#: GPD125-4 Mfg#: GPD125-4
      Part Number Description Type Substitution
      B1141605 TRANSFORMER 120V TO 24V, 40 VA F 0130M00140
      B1282628 GAS VALVE (36G) F B1282628S
      B1368047A BLOWER WHEEL 10 X 10 F
      B1370154 ROLL OUT SWITCH, 350º W/O BRKT. F
      B1370155 AUXILIARY LIMIT SWITCH 1200
      B1370819 DOOR SWITCH F
      B1401018S IGNITOR F
      B1859005S VENT MOTOR & HOUSING (GMP) F
      B2589342 #42 ORIFICE
      B4022700 INSHOT BURNER ASSY 3″
      CAP100000370VAS CAPACITOR 10/370V [BLOWER] F CAP100000370VAP
      0130F00010 FLAME SENSOR F
      1284835 CAPACITOR STRAP
      1370911S PRIMARY LIMIT SWITCH 220ºF F
      2508006P TOP PANEL 21″
      2510002S FLUE COLLECTOR BOX 21″
      2513503 BURNER RACK 5 CELL 0121F00159
      2514000 SIDE PLATE, BURNER BOX 0121F00146
      2515501 J-BOX COVER OBSOLETE
      2516506S WRAPPER 21″ OBSOLETE
      2524002 DUCT ANGLE 21″ OBSOLETE
      2528003S FLUE RESTRICTOR PLATE 2.250 DIA
      2539307S BLOWER SHELL ASSY 10 X 10 0271F00112S
      2568410 WIRE HARNESS, BLOWER 2578402S
      2568411 WIRE HARNESS, HEAT EXCHANGER 2578402S
      2568412 WIRE HARNESS, HSI MODULE 2578402S
      2568413 WIRE HARNESS, HSI CONNECT 4078401S
      4022703S BURNER RACK ASSY 5 CELL
      764741 BLWR BLK-OFF,10X10

      1. Thank you so much, Steve. Very much appreciate you taking the time to reply in detail and also for listing out the parts for my furnace. Do you also sell the tubing that goes with this pressure switch?

        Thanks again and wishing you a very happy and successful new year!


      2. Hi RD! Thanks so much for your kind words! I do not know what size of the inside diameter that the pressure switch tubing for your furnace uses. We sell some pressure switch tubing on the following pages: and Sorry that we do not sell any of the OEM pressure switch tubing. I would like to suggest that you take a piece of the tubing to your local auto parts store or Home Depot and let them try to match the size up for you. I hope you have a great day and weekend! Steve

  15. Hello Mr. Arnold , in looking for the cause of a Nordyne furnace not turning on I found the voltage to the draft infucer to be at 193 volts.Is this normal?

    1. Hi Abel! No sir, 193 volts would not be normal voltage going into power the draft inducer unless you are working on a commercial furnace that is powered by 220 volts. Most residential furnaces run off of 120 volts. You might have a bad draft inducer or control board. Something may be shorted out and causes the voltage to be high. We have lots of furnace troubleshooting advice on the following page: I would suggest going through the troubleshooting flow chart to see what is wrong: We also have lots of troubleshooting questions and answers on the following page: If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem. Steve

  16. Hello Steve,

    Thank you for all this information!

    When I call for heat, the inducer motor kicks in, and the pressure switch closes, but there is no voltage across the terminals (which I understand should be 24VAC). I’m assuming this is why the hot surface igniter isn’t getting a signal to turn on. Do you know how I can further diagnose if this is truly a pressure switch problem or a problem with the control board (or something else)? It just seems odd to have the pressure switch close and not have 24VAC. I have an 80% efficiency unit.

    1. Hi Jon! Yes, that is not normal for the pressure switch to not get 24 volts AC between each terminal to ground when the draft inducer is running. I would think that you either have an open safety control like a limit or rollout switch, a loose connection in the wires or a faulty control board. I would suggest testing the limit and rollouts to make sure all of them are closed allowing current to run through them. I hope you have a Happy New Year! Steve

      1. Steve,

        Thank you so much for your help and quick response. I went to recheck continuity over the limit and rollout switches, and they were all fine. I even checked the wires to make sure they were good. I didn’t have to replace or reset the switches, but when I plugged everything back in the furnace booted up!

        Happy New Year and thanks again! You are awesome.


      2. Great Jon! Fantastic! Glad to hear that your furnace started working! Thanks for taking your time to let me know! I hope you and your family have a Happy, Healthy and Prosperous 2019! Steve

  17. Dear Mr. Arnold, thank you for your valuable info here. I have a York 95% high efficiency furnace that has the same error code as one of the previous posts ( Jack from January 2017): 6 red flashes, means “while the unit was operating, the pressure switch opened 4 times during the call for heat. Check for faulty inducer, blocked vent pipe or faulty pressure switch. The furnace will lock out for one hour and then restart.” I checked the pressure switch using your method: ground on bare metal which is the switch box, the other on pressure switch, they read 24 volts, resistance by putting two probes on the pressure switch read almost nothing, so I don’t think my pressure switch is bad. I noticed the vent pipe (outgoing one) has one opening conneted to draft inducer with a rubber tubing, and the other opening is closed with a plastic cap. Every time the furnace runs, water will leak out from the cap to the bottom of the chamber. There is always a small pool of water after the furnace run. If I unplug the tube and the cap, water will run out of there, quite a bit. I blow air to those holes and from the tube to draft inducer, they don’t seem to be blocked but I hear water sometimes stays in draft inducer but not always. The condensation box connecting to the inducer seems working too. What do you think the problem could be? Is my vent pipe blocked? How can I check that? There is another thing I noticed that my draft inducer is having a quiet humming sound when furnace is not running, my other furnace (I have one for each floor) has no noticeable sound at all. Is this a concern too? Or is it related to my 6 red flashing code problem? And one more problem, my thermostat connected to this problem furnace seems starting to call heat way early than it should be, like reaching the designated temperature two hours early, like I set temperature to be 68 at 7 am, it’s already there by 5. I am not sure if these are all related or separate problems. Thank you for your advice. Sue

    1. Good morning Sue! I would suggest that you test the pressure switch with the furnace draft inducer running with the thermostat calling for heat. You should get a constant 24 volts between each of the two terminals of the pressure switch to a good ground on the furnace. Of course, the problem could be occurring when you are not testing with a voltmeter. Since you say that you are having water leaking inside the furnace this would be a sign that there is a leak in the vent system collector box or the furnace’s condensate drain is partially stopped up on your furnace. I would like to suggest that you disconnect the pressure switch tubing and clean the condensate drain on your furnace to make sure the drain system on your furnace is clean. Try to pour water into the draft inducer to make sure that the condensate drain line is open. You might have to attach a tube to the draft inducer so you can pour water into it. Do not use a vac with the pressure switch connected or it could damage the pressure switch. Do not blow or suck on the pressure switch with your mouth because this could damage the pressure switch. The draft inducer should not be humming and more than likely this is the low voltage transformer that you hear humming and not the draft inducer. The thermostat should not be connected to the 6 red flashes problem. The only way that I know to clean a vent pipe on a furnace is to take the plastic vent pipe apart and blow it out or suck it out with a strong wet vac. Most of the time the vent pipe being stopped up is not the problem. If it was me I would make sure the condensate drain, draft inducer drain is open on the furnace and if that did not work I would try a new pressure switch. I hope you can fix this problem soon. Hope you have a great day and week! Steve

  18. Hi, my pressure switch on the clare megasave has two tubes, i assume one is negative pressure and one positive pressure. If i disconnect the line with the positive pressure, the pressure switch shuts off, but with the disconnected tube it works fine. What could that be?

    1. Hi! Sorry, but I do not know what the problem could be. I am not familiar with Clare Megasave furnaces. I would suggest making sure that both ports where the pressure switches hook up are clear and open. If the furnace produces water make sure the drain is clean and draining properly. Make sure that the draft inducer vent pipe is clean. clear and the draft inducer is running up to full speed. If all these check out then I would recommend a new pressure switch. Sorry that I can not be much help. Steve

  19. i have a lennox rooftop the draft inducer will not start when stat is calling for heat if i jumper the air proving switch inducer and heater will come on and stay on for the reqd/ time but will not come back on again until i repeat the same sequence again and once again will run until the reqd temp is reached any input will be helpfull thank you jacob

    1. Hi Jacob! I am sorry that I do not know much about Lennox rooftop equipment. It sounds like you either have a bad pressure switch or control board since everything works when the pressure switch is jumped. Of course, you know that the draft inducer should be started by the control board when the thermostat calls for heat. I am guessing that the control board is at fault because the draft inducer will not start until the pressure switch is jumped. I am sorry that I can not be much help on this. Maybe someone reading this can be of more help I hope. Steve

  20. Hi Mr. A,
    Well here I am once again. I have a 28 year old Rheem (unit#2) RGVC 07EA – ER that is making me batty. By the way unit one is running find thanks for all the tips. Unit #2 runs perfect cycles with a jumper switch from the pressure switch NC to the NO poles. Thermostat calls for heat, starts the inducer motor, then without the jumper in place the unit inducer just runs with no ignition. I get 24Vs to the white wire on the PS but the black wire, that goes from the PS to the Honeywell S86F controller, reminds without power so no ignition. When I jumper the NC – NO poles the furnace works flawlessly. Calls for heat, runs a full heat cycle with the blower working as it should, until the thermostat is satisfied, then the blowers runs on the cool down cycle then shuts off until the next call for heat. Runs flawlessly without fail. New OEM switch just installed with new tube. I was taught the old method of mouth to tube and suck lightly to close the PS switch. The inducer fan is a Fasco replacement fan put in some years ago. If I do the mouth to tube test the furnace ignites and runs as long as I hold pressure on the switch. Hence my question. I thought that if the switch was jumped and the furnace ran a complete cycle that once it called for heat again the jumped poles would not allow the furnace to work because they were jumped, unless all tested out ok before the heat cycle began. I think my thinking is wrong but I am not quite sure. Wouldn’t the jumped switch prevent this re-ignition sequence from occurring? My need is due to the fact that I have had so many licensed and qualified professionals come, replace parts, charge rather large fess to no avail.
    Rev J
    P.S. Sorry to pester you so much but I am a sought of hands on guy who likes to solve mysteries.

    1. Hi Rev John! Yes sir you are right on most furnaces. I do not know for sure about Rheem, but the control board should not allow the furnace to run more than one cycle with the pressure switch jumped. On most furnaces you would get a “pressure switch stuck closed” fault on the control board. The pressure switch should open and close between each cycle. Maybe the control board you have is not designed to do this. Sorry that I do not know. The draft inducer as you know should be producing enough negative pressure to keep the pressure switch closed. I am sure you have all ready done this, but please make sure the hole in the draft induce is open and free of any obstructions. Please make sure the vent pipe is not obstructed. Please make sure the draft inducer is fastened tight to the furnace body. If you want me to try and look up parts then please send me your model number and serial number to our account. Sorry that I can not be of much help. Steve

      1. Can I tell by how often the bubbles are moving threw the hose and how often?Trying to decide if I should call for service

      2. You should not have bubbles running through your pressure switch tubing. Furnaces have air pressure switches and if you are getting water in the tubes you might have a stopped up furnace condensate drain if you have a 92% condensing furnace or moisture might be getting into your furnace vent system. Liquid should not be in the pressure switch tube. I hope you can easily find and fix the problem. Steve

  21. Arnold, I am troubleshooting a York GY9S080B12UP11J 90% furnace that is throwing a “Pressure switch cycle lockout” fault code. The User’s Information, Maintenance and Service Manual, , expands on this: “6 RED FLASHES: This indicates that after the unit was operating, the pressure switch opened 4 times during the call for heat. If the main blower is in a ‘Delay on’ mode it will complete it, and any subsequent delay off period. The furnace will lock out for one hour and then restart.”

    I teed into the hose that goes from the port on the draft inducer to the pressure switch with a Bourdon style vacuum/pressure gauge, the kind sold in auto parts stores (don’t yet have a manometer, it’s in the mail). With the call for heat sequence initiated the draft inducer motor started up. There was no discernible movement in the gauge needle, but I could hear the pressure switch click closed. I realize the vacuum gauge is not quite fine enough to get an accurate reading, but I thought I would see SOME movement in the needle. I suspected that the negative pressure at the draft inducer housing port was weak, so I checked the usual suspects. There was no problem with the condensate drainage, the motor ran smoothly and the combustion gas vent was perfectly clear, so I took apart the draft inducer housing. Everything looked great. The wheel fan was in perfect condition and there were no cracks in the housing. Could there be a problem in the heat exchanger, or some other condition, that would cause a weak negative pressure at the draft inducer housing port?


    1. Hi Jack! Any leaks in the heat exchanger or your combustion chamber could cause a weak negative pressure at the draft inducer housing port. You might try to suck out the condensate drain with a wet vac to make sure there is not an obstruction in the drain or in the secondary heat exchanger. I am sure you already know this, but please disconnect your pressure switch before using a wet vac on the furnace because this will damage the pressure switch. Do not suck on the pressure switch with your mouth to try and produce a vac. This can damage the pressure switch too. I looked it up in our York parts program and your furnace uses pressure switch part number: S1-02435261000. In the description is says, “SWITCH,PRES,AIR,1.00 ON FALL,SPNO”. It looks like it is a 1.0 WC pressure switch. You might try a new pressure switch if all else fails. We do not stock this pressure switch. I hope you can easily find and fix the problem. Steve

      1. Just received the manometer I ordered. Going to check the negative pressure at the draft inducer port tomorrow. Tracked down the service manual/parts list for this model and found the pressure switch on eBay and bought it just in case. Will post findings.
        I checked the condensate drainage system. I removed and blew out the trap, reinstalled and observed the condensate dripping out while the furnace ran. There are also no problems with the vent.
        The manometer arrived, and measuring between the draft inducer and the pressure switch for 15 minutes with the manometer teed in, I got an average reading of 1.35 inches of water column with a low reading of 1.22. The pressure switch is rated to open at 1.00 inches of water column. The error code that keeps popping up indicates that the pressure switch closed four times during the last call for heat. I have read that the negative pressure produced by the draft inducer should be 3 times higher than what is required to close the pressure switch. This is obviously not happening in this case. This a case of low negative pressure at the draft inducer, that is not caused by a drainage or venting problem, or by defecting hoses to and from the pressure switch. I checked all that.

        I can only conclude that the problem must lie with either the draft inducer itself, defects in the heat transfer system or, least likely, the pressure switch.

      2. Thanks Jack! I am interested to know what you find the problem is.

      3. To day I found a double trap condition caused by the condensate drain line being too deep in the well of the condensate pump. I cut the 1 inch PVC condensate drain line where it enters the condensate drain pump so that the bottom of the drain line is never below the water in the pump well. I am waiting to hear back from the client to see if this fixed the problem. Will let you know the outcome.

      4. Hi Jack! Thanks for sharing! I hope this fixes your customers problem. I can see where if the drain tube was bottomed out against the bottom of the condensate pump that this would cause the furnace drain to stop up.

  22. Lots of good info here. Quick question…having a similar issue. Thermostat calls for heat, inducer comes on, pressure switch closes, burners ignite, etc. However, when the blower motor kicks on, the pressure switch temporarily opens and of course the burners drop out. I’ve checked the vent tube, vent, etc. Now for the weird part…I noticed the air filter was stopped up. Removed it and now the system is working correctly 90% of the time. Any suggestions? What would the air filter matter? The induction motor, hose, etc seem to be fine. 20
    Year old Lenox unit.

    1. Hi Mr. Smith! This could be a number of things causing this problem. It could be a voltage drop in the low voltage circuit when the blower fan relay on the control board kicks on. I would suggest that you test the voltage going to the pressure switch when the blower comes on. You should be above 24 volts AC when the blower kicks on. If you aren’t then you might have a contact in the control board that is arcing, getting weak and causing a voltage drop. I really have no idea why the problem would get better when the filter was changed. Please make sure that the draft inducer is securely attached to the furnace body with no leaks. You might want to check to make sure the inducer is tight. Please make sure you do not have any leaks in your return air system. If the return air is open anywhere near the furnace it could be causing negative pressure in the furnace room and can cause the pressure switch to not open properly. Sorry that I can not give anymore advice. I hope you can easily find and fix this problem. Steve

  23. i read your writing here,very helpful , thanks for further consideration excelent

  24. I have an older Payne 373LAV036070 that is having problems starting and staying on in the case it does start. The sequence of events I am observing is: 1) blower starts and runs for about 1 minute or so, 2) the blower turns off and the draft inducer starts, 3) the igniters glow, 4) The flame turns on and blower starts 5) heater runs for a couple of minutes (draft inducer and blower going), 5) draft inducer goes off, flame goes off, blower continues 7) blower shuts off, 8) cycle starts over, sometimes, but rarely successfully. When it fails, LED code 31 flashes, indicating a pressure switch failure. There is some buzzing from the controller that seems to coincide with the call for heat. I have to pull the plug to reset and start the cycle again. (BTW, I get about 26 volts between W & C.)
    From the things I’ve read, the blower shouldn’t start before the draft inducer. If that is a symptom, then I suppose it’s a controller issue. If not, maybe it’s something to do with the pressure switch or an obstruction in the draft inducer ducting. Two questions…. What do you think? And, can I still get the parts to fix this unit?

    1. Hi Greg! Sorry you are having trouble with your furnace running the complete cycle. I will start by saying that if you turn the power switch off on the side of your furnace then turn the power back on it is normal for the furnace to run for a minute or 2 to get rid of any heat that is stored in the furnace in case of a power failure. This 1 to 2 minute run time is built into the board and is normal. If the blower starts from the very beginning without turning the power off and on then yes, I agree you probably have a control board problem. From your description it sounds like the furnace is running normal until it cuts the gas off prematurely. This is probably because the furnace has over-heated and is going off on limit. When the furnace cuts off prematurely please check between W and C (com). You should get a constant 24 to 28 volts AC from the thermostat between these two terminals on your control board. If not then the thermostat is cutting the gas off. If you are getting 24 to 28 volts then this is normal and I would suggest troubleshooting the next control which would be the limit. You should be getting 24 to 28 volts from each terminal on the limit to a good ground. We discuss this problem of over-heating on the following page: If you would like for me to look up parts for you then please send your furnace’s product number. There should be 4 letters after the model number 373LAV036070. It would be best to contact me at the email address if you want me to recommend parts. Please let me know if I can help you out. I hope you can easily find and fix this problem. Steve

      1. Thank you for the quick reply. First thing…you are right about the blower starting in the beginning. It seems to coincide with my constant resetting to evaluate the symptoms. So, the scenario I gave you was what I thought was the most telling, because it went most of the way through a cycle without shutting off. However, often the furnace fails to start. I am consistently getting 26 volts between W & C (and have even bypassed the thermostat for testing.) The limit switch shows voltage on both terminals between ground and no resistance between them. When the flame does come on, the cycle ends with no resistance between the terminals. The one thing that seems constant in all cases is the draft inducer is shutting off as the first sign of failure. It appears the pressure switch is working ok… both terminals have 26 volts (when grounded) past the point where the draft inducer shuts off, only closing once the motor slows a bit. I think I am also hearing the switch close a few seconds after the inducer shuts off. The manually re-settable switches don’t appear to have triggered. Knowing nothing about this stuff (but a little about other stuff), I would guess there is a faulty relay meant to to trigger the inducer motor. Maybe this is just part of the controller. I really appreciate your help on this. Thank you, Greg

      2. Hi Greg! Yes, you would need to see what is controlling the draft inducer, either a relay or a control board. I would suggest tracing the wires from the draft inducer to see what is supplying the power to the draft inducer either a relay or control board. This is probably where the problem is. Either the draft inducer is bad or the relay or control board relay is bad. I hope you can easily find and fix the problem. Steve

    2. Perhaps the ps has a pin hole in the diaphram

      1. Yes, The pressure switch could very well have a pin hole in the diaphram.

  25. Im having the same issue. Get 3 blinks indicating the switch is stuck open. Checked all my vents and they are clear. My inducer fan starts but wont trigger my switch closed. If i suck on the hose, it closes, glow coil glows, and igniter ignites. But as soon as i remove my mouth from the hose it all shuts back down. I borrowed a manometer from a buddy and tested on the inducer fan when it starts up. Its reading .38-.46 in-wg. Not sure exactly what that means. But i know its telling me that the fan is sucking in air. The furnace worked this past April. Any help is appreciated. Its a tempstar furnace. 80 afue. Installed around 07-08.

    1. Hi Mr. Backus!
      This sounds like you need a new pressure switch. I heard recently that you should not suck on the pressure switches because it can mess them up. I used to do this so do not feel bad. Please send me your furnace’s model number and I will tell you which pressure switch your furnace is supposed to have. Please email me at with the model number or manufacturer’s number. I hope you have a great day! Steve

  26. my gas unit code 3 flashes my pressure switch not work , i replace the pressure switch nothing happen my reducer fan didn’t come on my glow bulb didn’t light up please tell what the problem.

    1. Hi Mr. Arnold! Great last name! If the draft inducer does not work the pressure switch will not close and send the message to the control board that the pressure switch is not closing and you will get the 3 flashes. Your main problem could be the control board or draft inducer. When the control board receives 24 to 28 volts AC from the thermostat between terminals W and C (com) the draft inducer should start. Please check and make sure you are getting 24 volts AC between W & C. Then test to see if you are getting 110 to 125 volts AC to the draft inducer. If you aren’t then probably the control board relay on the control board that controls the draft inducer is bad. If you are getting 110 to the draft inducer and it is not starting then you have a bad draft inducer motor. We have a troubleshooting flow chart on the following page the should help you find the problem: If you want me to look up parts please send me your furnace’s model number. I hope you can easily find the problem. Steve Arnold

      1. i have kenmore gas furnace 80 ,it stopped a week ago .a friend came,he started the furnace and pressure switch was the problem .the furnace stopped again . if this is switch problem , can we put a compatable switch old part # is fs 5070-52.

      2. Hi Mr. Chaudry! Please send your Kenmore furnace model number to me at our address and I will try and look it up and see which pressure switch is used on your furnace. Please make sure that your vent is open and the hole where the pressure switch tube is attached is not stopped up. I would suggest taking the pressure switch tubing off and running a wire or a small drill bit in through the hole in the draft inducer to make sure it is open. I hope you can easily find and fix the problem. Steve

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