Posted on 116 Comments

How to test and troubleshoot gas furnace pressure switches.

Furnace

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

Answer:

  • 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 the 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 with the draft inducer running you should be getting 24 or more volts between both leads to the 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 the 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 draft inducer running then you have a pressure switch problem.
  • Please click here if you are interested in seeing the furnace pressure switches that we sell
  • 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. A weak or bad draft inducer run capacitor could cause the draft inducer to run slow.
    • Your draft inducer could be loose and not attached tightly to the body of the furnace. Make sure that the draft inducer is attached tightly with no air leaks.
    • You could have a bad control board that is not sending 110 volts to the draft inducer to allow the draft inducer to run. You might have a bad control board draft inducer relay.
    • 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 where the pressure switch tube connects 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.
    • In high-efficiency condensing furnaces, the water drain line could be stopped up, causing 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 as we advise on this page. I wanted to pass this along so others will not damage pressure switches.
    •  A leaking heat exchanger can cause the pressure switch to not close because when the blower comes on the blower pressure is allowed to mix with the combustion air and will mess up the pressure switch’s negative pressure. A hole in the heat exchanger will not allow the draft inducer to create enough draft (negative pressure) to close the pressure switch. A stopped up heat exchanger will not allow the combustion air to circulate properly and cause the pressure switch to open up. I hope that you do not have a leaking heat exchanger because most of the time this means a new furnace.
  • Please click here if you are interested in seeing the furnace pressure switches that we sell
  • Please email us anytime if you have any questions. Please send us your furnace’s model number if you would like us to recommend a pressure switch or part for your furnace. Our email address is Support@arnoldservice.com  We would love to help you out and earn your business! 
  • Below we have three really good Youtube videos made by GrayFurnaceman and Word of Advice TV that show how to troubleshoot pressure switches on an 80% furnace and how to by-pass a furnace pressure switch. Thanks to GrayFurnaceman and Word of Advice TV for making these informative videos! Jumping or by-passing a pressure switch would be good to do if you were troubleshooting and wanted to make sure that the pressure switch was the problem.  I hope these videos will help you troubleshoot your gas furnace pressure switch.

Please feel free to search our site for More Troubleshooting Problems with Solutions, Heating, Air Conditioning Parts and Supplies in the Google Seach box below.

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

  1. Hi Steve,
    I have a Rheem model # RGRS-09EZAGS. I am getting a two blink coed that states. “pressure switch stuck open, switch will not close.” I have taken the intake piping apart as much as possible and cannot find any restriction. I cannot get the code to default when the induction cover is removed from the furnace. I am assuming that would mean there is an intake restriction or the pressure switch is too sensitive. I do not believe the plumping is the problem since this has developed over time. The piping is 3″, but it does reduce to 2?” when it gets to the furnace. The furnace is about 10 years old. The problem seems to worsen when the temperature is colder. If I leave one screw undone on the induction cover so that the cover is not sealed to the furnace case, the furnace functions properly.

    1. Hi Tim! Sorry to hear that you are having trouble with your pressure switch not closing. This means most of the time that the inducer is not being allowed to draw enough negative pressure. We discuss this in another post that we have on the following page: https://arnoldservice.com/problem-furnace-blink-code-shows-i-have-a-pressure-switch-problem-replaced-pressure-switch-still-have-a-problem/ You could have a partially stopped up secondary heat exchanger or a drain that is partially plugged up. You could need a new pressure switch. I would suggest making sure the draft inducer is fastened tight against the body of your furnace and make sure your drain is open. I have a screenshot here that shows the pressure switch that your furnace uses: https://www.screencast.com/t/cWVlOrh5R I believe that this is the upgrade pressure switch that Rheem is recommending in this screenshot: https://www.screencast.com/t/04eJYSW3R I do not know what they mean by a 278 option. I hope you find a simple and inexpensive fix. God bless you. Steve

  2. I’m trying to fix my AS Freedom 80 (model AUD080R9V3H1) and it looks about identical to the furnace he’s demonstrating on in World of Advice. Should the hi pressure switch be tested the same as the lo? Should there be 27VAC and then 0V across the terminals? Because while I’m getting 27V across the lo switch, I’m only getting from .6 to 1.6V across the hi switch. I’ve tested that voltage at the switch and also down at the controller connector. The inducer fan is spinning, the tubes aren’t blocked, both switches show they’re closing with the inducer fan running. Also, the controller (White-Rodgers 50A51-507) sometimes blinks 3x showing a pressure sensor problem, and sometimes shows solid red (failed controller). Sometimes the furnace runs long enough to light and then shuts right down, sometimes it only gets as far as tripping the pressure switches and then purges. Is the controller bad?

    1. Hi Randy! With the furnace is low heat and the draft inducer running at low speed the low heat pressure switch should be closed and you should get 24 volts from each terminal of the pressure switch to a good ground. In high heat with the draft inducer running at full speed, you should get 24 volts between both terminals of the high heat pressure switch to the ground. Make sure that your furnace condensate and furnace vent are not stopped up if you are having trouble with your pressure switches working correctly. I would like to suggest that you make sure that all safety controls on your furnace are closed like the rollouts, limit, and pressure switch. If all the safety controls are closed and allowing current to flow through them and you are still having trouble then you might need a new control board. Trane, American Standard recommends the KIT09370 control board as a replacement control board. Please see the picture I copied from our parts program on the following page: https://arnoldservice.com/wp-content/uploads/2020/12/Trane-furnace-parts-AUD080R9V3H1.jpg I am sorry we do not stock that control board kit. I hope you can easily find and fix the problem. Steve

  3. We have 2 American Standard Freedom 80 furnaces, model ADD080R936E0. One of them is refusing to turn on. When the thermostat calls for heat, the status light indicates a call for heat, the flue draft blower comes on, and the ignitor glows bright orange. After about 10 seconds, the ignitor shuts off but the blower does not come on. After about 45 seconds, the cycle stops and starts again. After 2 tries, I get the error code “External lockout (retries or recycles exceeded)”.

    I checked out the low side pressure switch like you show in your video and it looks fine. The fact that I hear a relay close about 10 seconds after the inducer blower starts as the good furnace lights the burner but in the bad one I hear nothing makes me think it’s the control board. Would you agree?

    1. Hi Steve! Yes, I would agree that it sounds like you have a control board problem if all of your safety controls are closed and your gas valve is not getting 24 volts to allow it to open. It sounds like it could either be the control board or gas valve. We have a new post about checking all the safety controls on the following page: https://arnoldservice.com/problem-how-do-i-test-my-gas-furnace-rollout-switch-limit-switch-pressure-switch-and-flame-sensor/ There is a really good Youtube video on troubleshooting gas valve problems on the following page: https://www.youtube.com/watch?v=0zKl9wvNIRk If all your safety controls are closed and the gas valve is not getting power to open and supply gas to the furnace then I would think that you have a control board problem. If you would like for me to look up any parts for your furnace then please send me your furnace’s model number to our email address arnoldservice@gmail.com. I hope you can easily find and fix the problem. Steve

  4. I have 17 y.o Bryant furnace which 2nd exchanger was recalled.
    I visually checked primary and secondary from outside – look good.
    I am afraid to miss signs of clogged/rusted the entry from primary to secondary – when roll out will happen.
    I did check the actual pressure in the entry to the pressure switch = 3.05 inH2O. (it will probably go up before the secondary gets completely clogged and the flames will still look normal blue).
    Will soon get the tool and will measure the CO PPM in exhaust.
    Question.
    What are the ‘normal’ ranges of these measurements?

    1. Hi Alex! I am sorry, but I do not know what the acceptable measurements for CO in the exhaust of a 92% furnace. I have not done any service since 2005. I Googled your question and found the following statement. “47 standard (1998) requires that an air-free flue gas sample of CO not exceed a maximum of 400 ppm (0.04 percent) when a furnace vent is either partially or completely blocked.” Here is the link when I Googled your question: https://www.google.com/search?q=What+are+the+proper+carbon+monoxide+reading+in+the+vent+of+a+92%25+gas+furnace%3F&rlz=1C1ASUM_enUS497US497&oq=What+are+the+proper+carbon+monoxide+reading+in+the+vent+of+a+92%25+gas+furnace%3F&aqs=chrome..69i57.31718j1j15&sourceid=chrome&ie=UTF-8 I hope this helps you out. Steve

  5. Hi
    Hope you are well. Can you please give me an idea of the cost of purchasing a Honeywell pressure switch H fitting for my TRANE fully modulating furnace? As the part number model is 50014119-003. I contacted the supplier in Coquitlam BC. National Energy and Supply. They told me this morning they can’t give me any pricing. Can you give me an idea of what I should expect to pay for this part as a homeowner? It’s fair to pay twice the cost the guy installing it as a homeowner, he’s got to charge something extra….but I feel a right to know a benchmark, so I know in advance what I am being charged is fair…and I am not being ripped off…thank you. Teresa

    1. Hi Teresa! I am sorry, but we do not stock the H fitting that you are looking for. I am sorry, but I have not idea how much the part costs and the labor that would be involved in replacing the part. I am very sorry that I can not help you out with this. Steve

  6. Hi Mr. Arnold – Thank you for your informative site. I’m hoping for some of your insight. I have an older 80% gas furnace that won’t start up. The draft inducer starts up but then quits after a few seconds and the ignitor never lights. The error code claims pressure switch is open so I did some troubleshooting with my meter. I first checked to see if there is any voltage to the ignitor when the inducer revs up – nope. I then checked voltage to/from the switch. When the inducer starts up, I’m getting 24v to the inbound terminal of the pressure switch. I’m also getting 24v from the outbound terminal of the pressure switch. Seemingly this would indicate the switch if working properly. I traced the outbound wire back to the board and backstabbed the wire in the harness – no voltage! Is it possible the wire is bad and if so, is it possible to change only the one wire or does the entire harness assembly need to be changed? Thank you so much for your help.

    1. Hi Ricardo! If the draft inducer is starting up and then cutting off after only a few seconds then I would think that your control board is bad or your thermostat is cutting the 24 volts off. I would like to suggest that you test and make sure that you are getting a constant 24 volts between the W and C (com) connections on your control board when the thermostat is calling for heat to be ON. If you are not getting the 24 volts constantly then I would think that you have a thermostat problem. Make sure the batteries are in good condition if your thermostat uses batteries. If you are getting a constant 24 volts between W and C and the draft inducer is stopping then I would think you either have a control board problem or a draft inducer problem. I believe you are getting the pressure switch error message because the draft inducer is not staying on long enough to start a heating cycle. If you do need to replace one of the wires that are bad then I would suggest using wire nuts to fix the wire instead of purchasing a new wiring harness. I hope you can easily find and fix the problem. Steve

  7. Hi there, would you mind helping me out with a question I have? What’s your opinion on Makita drill drivers? Are they worth the price? Or do you think a cheaper brand is good like Ryobi? I just got a job where I need a Lithium Ion tool kit. I really appreciate it!

    https://www.tool.report/types/power-drills/

    1. I have a Dewalt and a Milwaukee and really like those two drills. I am sorry, but I do not know anything about Ryobi.

  8. I have a furnace that starts the heat cycle without any issues. After about 4-5minutes into the heating cycle I can hear a change in the sound the furnace makes when it is running. Then all of a sudden the pressure switch opens for 1/2 second (checked it with an oscilloscope) and then closes again. When the pressure switch opens the gas valve closes and the furnace starts another ignition cycle because the pressure switch closed after that 1/2 second glitch. The furnace then runs awhile longer and this crazy cycle repeats…pressure switch glitches open, gas shuts off, furnace starts another ignition cycle. If it is cold outside this cycle happens 4 times before the controller locks down the furnace. If it is mild outside the furnace is able to complete a heating cycle before the 4 glitch cycles happen… and the controller board is happy. The outdoor vents are cleared. I took out the condensate trap and rinsed it out – I can run a good stream of water into any of the 3 input lines and water flows out of the trap w/o issues. I pulled the burner chamber door off during a heat cycle and the same 4 glitch pressure switch openings happen. I am going to recheck the hoses to make sure they and the induction motor orifice are cleared. Any or suggestions ideas?

    1. Hi Gene! It sounds like you pretty much covered all the bases for troubleshooting. The question and problem is, what is causing the pressure switch to drop out. You could have a draft inducer motor that is failing or if your draft inducer has a capacitor then maybe the capacitor is week. I would like to suggest that you check the voltage going to the draft inducer and see if the voltage goes down momentarily when the problem occurs. If the voltage goes down then you might have a relay on the control board that controls the draft inducer that is going bad. I would make sure all connections are good and tight. I have seen the white plastic Molex connections that plugin where the plugins do not mate properly and cause poor connections with arcing. You might want to unplug and plug back in any Molex connections. I am sorry, that I can not give any other advice other than you might try a new pressure switch if this continues to happen. If you would like for me to look up parts then please send your furnace model number to our email address arnoldservice@gmail.com. I hope you can find and fix the problem soon! Please let me know when you find the problem so I can share it with others. Thanks! Steve

  9. Hi Steve. Greetings from MN. I have a RUUD furnace and the pressure switch was replaced in November after it was determined by the tech the switch was the problem. It just went out again last night, so I called the emergency service line to get it fixed. Each time it was replaced, the furnace has run properly afterward. If there were underlying issues such as obstructions, condensation line blockage, etc., would the furnace even run properly after a new pressure switch was installed? Guess what I am asking is more likely than not, I probably got a bad switch the first time it was replaced if it only lasted a couple of months, or, could there be another problem that would cause the switch to go out so quickly?

    Thanks,

    -Chris

    1. Hi Chris! Yes, more than likely you got a bad pressure switch. If you had an obstruction in the condensate line then water could get up into the switch and cause it to fail. I would suggest taking the pressure switch tube off so that it does not damage the new pressure switch and use a wet vac on the furnace’s condensate trap and line to make sure that the condensate line is not partially stopped up. The service tech should have done that. I know those pressure switches are not cheap! If your old switch had water up in it then you probably need to clean the condensate line. I hope the new pressure switch lasts for many years as they should. If you want me to look up the right pressure switch for your furnace then please send me your furnace’s model number and serial number to our email address: arnoldservice@gmail.com

  10. I have a Armstrong 80% advange V, my problem was and still is the pressure switch I had switch replaced by a furnace repairman week ago and i still have a problem can you tell about the two rubber hose on the switch and which one for inducer and which one for gas valve Thank You for any information

    1. Hi Lee! I would think that the two hoses attached to your pressure switch are that one senses the negative pressure of the draft inducer. My question would be did the service tech you had clean the draft inducer and make sure that the hole in the draft inducer was open so the pressure switch can sense the negative pressure the draft inducer creates when the draft inducer is running? You could have a partially stopped up vent, or a dirty draft inducer. The hole that goes into the draft inducer might be partially stopped up. You could have a leak in the draft inducer. I would suggest making sure that the draft inducer is tight against the body of your furnace. Check the draft inducer screws that secure the draft inducer to the body of the furnace to make sure they are good and tight. I would think that the other pressure switch tube that goes to the gas valve would serve as a gas valve vent. Any gas that is released by the gas valve will go into the draft inducer and out the vent stack. If your draft inducer has a capacitor, I would like to suggest that you make sure the capacitor is in good condition. A slow running draft inducer will cause pressure switch problems. I hope that you can find and fix the problem soon! Steve

  11. Mr. Arnold, I’ve got a good one for you. I replaced the Inducer blower motor on a Rheem Classic 90 Plus, model RGRA-09EZAJS. The old one was squeaky and then quit (lots of rust and corrosion). I purchased a Fasco replacement from a reputable parts supply store. Additionally, I replaced the capacitor and one of the rollout sensors. The furnace operated as it should prior to the motor going out. Here’s the problem. The system only functions if the top access door is off the unit. When I put it on, everything operates as it should, but when the lower blower motor kicks on and the flame burns out-every time. I’ve triple checked the wiring (took pictures before disassembling), ensured the vacuum tubes are unobstructed and the drain line drains when operating without the top access cover. Could a new inducer motor impact the pressure switches (pressure switch 42-24195-03 and air pressure switch 42-24012-05). Intake and outflow PVC pipes are unobstructed and I can feel air coming into the housing from the intake and have exhaust departing the out flow line. This has me stumped.

    1. Hi Scott! Sorry to say that this sounds like you have a heat exchanger leak because you say that everything is OK until the blower motor kicks on and I believe that the blower motor air is mixing with the heat exchanger air and causing the pressure switch to go out. I would like to recommend that you have the heat exchanger examined for leaks. This could also be caused by a leak in the draft inducer seal. Did you replace the seal on the new draft inducer or use high temp silicone sealant to seal the new draft inducer. That is the only thing that I can think of that might cause the problem you describe. Sorry that I can not be of much help. I will leave you a downloadable parts list PDF file in your email inbox for your reference. God bless you and your family. I hope you can find and fix the problem soon! Steve

  12. I have an old Moncrief conversion furnace. When the furnace was checked last spring, it was running at 97% efficiency. On New Year’s Day, it stopped working. The pilot is lit with a nice strong blue flame but the burner won’t come on. We checked the thermostat which was fine and changed the thermoregulatory. It flashed on for a second and went off but won’t come back on.

    1. Hi Wanda! Sorry to hear that you are having problems with your Moncrief furnace. I am sorry, but I do not know anything about old Moncrief conversion furnaces. If you want to send me your furnace’s model number I will try and look up parts to see what you have. You say that the pilot is burning but the gas burners will not light. I would like to suggest that you use a voltmeter with the thermostat turned up where the thermostat is calling for heat. Make sure that you are getting 24 to 28 volts between the W and C com wires. If you are not getting 24 volts between these two wires then you have a thermostat or thermostat wiring problem. I would like to suggest that you turn off the furnace and clean the pilot with compressed air or a Vac. While the furnace is turned off check all wire connections and grounds to make sure they are good and tight. We have lots of furnace troubleshooting advice on the following page: https://arnoldservice.com/gas-electric-furnace-troubleshooting-simplified-2/ I would suggest going through the troubleshooting flow chart to see what is wrong: https://arnoldservice.com/wp-content/uploads/2014/12/furnace-troubleshooting-flow-chart.pdf We also have lots of troubleshooting questions and answers on the following page: https://arnoldservice.com/furnace-troubleshooting-questions-answers-index/ If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem.

  13. i have a therma pride CHB1-100n that is showing 2 flashes for pressure switch stuck open. I’m assuming pressure switch needs replacing? no blockage on intake or output.

    1. Hi Ed! Yes, sounds like you probably need a new pressure switch. You can temporarily join the two wires together that go to the pressure switch to make a complete circuit and get one heating cycle out of your furnace. The problem is after each heating cycle you will have to separate the two wires and allow the circuit board to sense the pressure switch has opened back up before you try another heating cycle. I am sorry, but I do not have a Therma Pride parts program where I can look up parts for you. You might try Repair Clinic for a new pressure switch. I hope you have a great day and a Happy New Year! Steve

      1. it was lighting for just a second then going out & throwing the 2 flashes, now it’s not even trying to light just the igniter lights up. I ordered a pressure switch so hopefully that’s the problem. Thanks

      2. What does it mean when you refer to the condensate drain is stopped up? does it mean plugged up?

      3. Hi Ed! On most 90% High-Efficiency furnaces they have condensate that forms during combustion in the secondary heat exchanger. If you have a high efficiency furnace then you probably have a condensate drain and if the condensate drain becomes clogged up it will cause pressure switch problems. You can disconnect the pressure switch tubing so it will not get damaged and suck the condensate drain line out with a wet vac. I hope the new pressure switch fixes your problem. God bless you and your family today and always! Steve

  14. 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?

    Thanks

    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

  15. 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.

      Cheers,
      Rajesh

      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]
        MPL-9300-V-0.4-DEACT-N/O-VS
        D/C 31/99/CR

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

        Thanks,
        Raj

      3. Hi Raj! I answered you through your email address and ours arnoldservice@gmail.com. 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

  16. 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.

        Cheers,
        Raj

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

  17. 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

  18. 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 arnoldservice@gmail.com email address. I hope you can find and fix the problem soon! Steve

      1. Hi Steve. I really appreciate all of your posts. The information is really valuable. I have a similar issue as this one above on an older Payne, but not exactly the same. I’ve gone through the recommended steps of testing the pressure switch, wet vac on the trap and drain lines (with the pressure switch tubes disconnected), and making sure all tubes are clear. From disconnected power, after the blower cycle, I get error code 31 – the dreaded pressure switch fault. Through multiple trial and error, I’ve found that if I pinch the tube connected to the pressure switch just above the T connection, on the piece of tube leading to the combustion chamber, I can get past the switch test, and get the furnace to fire and run (for around 20 minutes). Otherwise, I don’t get to the point where the ignitor clicks on and so forth. So, two questions: why would plugging the combustion chamber feed to the pressure switch allow the pressure to reach the desired level (and why not when it is not plugged / pinched) ? Also, would it be ok just to leave this little hose pinched closed?

      2. Hi Greg! Thank you so very much for your kind words! I would not recommend pinching the pressure switch tubing unless you are freezing and in an emergency situation where you need the heat really bad because as you know the pressure switch is a safety device and the problem needs to be found out and corrected to be safe. There are several things that can cause pressure switches not to close properly and I have them listed in this post. Maybe your vent is partially stopped up or you could have a heat exchanger leak. I hope not! Something is robbing the negative pressure in your furnace and you would need to find the problem. Sorry that I can not be of much help. Steve

  19. 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: arnoldservice@gmail.com 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

  20. 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.
    Tracy

    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

  21. 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 arnoldservice@gmail.com. I hope you have a great day! Steve

  22. 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 arnoldservice@gmail.com. I hope you can easily find and fix the problem. Steve

  23. 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

  24. 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 arnoldservice@gmail.com. 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

  25. 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: arnoldservice@gmail.com 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?