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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 ground. 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, 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 you 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 vacumn 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 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 volt meter 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: Support@arnoldservice.com
  • 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.

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

  1. 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

  2. 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 arnoldservice@gmail.com 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

  3. 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, https://goo.gl/qIHBqn , 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?

    Thanks,
    Jack

    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.

  4. 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

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

  6. 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: https://arnoldservice.com/problem-furnace-comes-doesnt-run-long-shuts-off-will-work-thermostat-turned-way/ 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 arnoldservice@gmail.com 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.

  7. 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 arnoldservice@gmail.com with the model number or manufacturer’s number. I hope you have a great day! Steve

  8. 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. mrbiggzforever50@gmail.com

    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: https://arnoldserv.wpengine.com/furnace-trouble-shooting-flow-chart/ 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 arnoldservice@gmail.com 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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