Posted on 66 Comments

Furnace Troubleshooting flow chart

This gas furnace troubleshooting flowchart should help you troubleshoot and see what is wrong with your furnace. Just follow the steps. You will need a volt meter for testing. The picture below is pretty small, especially if you do not have any zooming capabilities with your computer. We have a link for a downloadable PDF file if you want to print the chart out so you can read it and take it with you while troubleshooting. Please click on the following link if you would like the downloadable PDF flow chart.: furnace troubleshooting flow chart If you have any questions please email us anytime. Our email address is: support@arnoldservice.com  We would love to help you out and have your business!

 


66 thoughts on “Furnace Troubleshooting flow chart

  1. Hi Steve, great column and advice and hope you may have some insights on my issue. Lennox GH19 propane furnace. Last season remember the ignition clicking sound was taking longer. This year just clicking cycling and no ignition. First tech came out and diagnosed that it needed a new circuit board, So I said OK do the work. Next tech brought a new Honeywell S8610U ignition control and a new Honeywell ST9120U universal board to replace the ST9120B 1005 board. Tried two different ST9120U boards, could not get it to work. Mentioned there was some minimal voltage detected ~5v? but not the 24v needed. So he is stumped and trying to check with others. Pressure switch doesnt seemed blocked, limit switch in cabinet has continuity. And other parts supposedly checked.

    1. Hi Steve! This sounds like your furnace is sparking (clicking) but there is not any ignition to the pilot. Maybe no gas to the pilot. If you have a gas valve that has a PV (pilot valve) and an MV (main valve) then I would think that either you are not getting the 24 volts to the PV since the pilot is not lighting to continue the furnace ignition sequence. I would like to suggest that you test with a voltmeter to see if you are getting 24 volts to the PV if not then you might have a bad control board, wrong control board, or an open safety control (pressure switch, limit, or rollout) that is not allowing the pilot to light. If you are getting 24 volts to the PV and the gas valve will not light the pilot then I would think that you have a bad gas valve. If you want me to look it up and see which control board that Lennox recommends for your furnace then please send me your furnace’s model number to our email address arnoldservice@gmail.com and I will be glad to look it up and give you the recommended Lennox control board part number. I hope that you can easily find and fix the problem. Steve

      1. Steve, yes PV MV gas valve emailed a photo of the Lennox unit I have and its control board and the gas valve. The tech doesnt seem to have 24v going to gas valve, so he didnt think it was the valve. The puzzling part was the Honeywell universal replacement board didnt seem to be sending out a measureable 24v to the gas valve? And he tried 2 new boards. So yes curious to see what board you show for it as a replacement as the unit is in ok shape except for this issue and winter is a coming.

      2. Hi Steve! I might be the ignition control board and not the fan control timer that I sent information to you on through our email. Yes, if the gas valve is not getting 24 volts then you probably have a control board problem or an open safety control that is not letting the gas valve get the voltage to open. I hope you can find and fix the problem soon! Steve

  2. Hello Sir!

    I have a Goodman GMP050-3 80% furnace. The furnace in the basement that heats the basement and ground floor. On really cold nights in the single digits, the heater will start normally and will turn off once the temp is reached at the thermostat. Then between 5 and 15 minutes, the furnace will repeat the cycle all night without fail for 18 years. About a month ago the furnace would light once but then not turn on again until we lower the thermostat, turn the switch off next to the furnace and let it rest for about 30-60 minutes. It will then successfully start and run. When it malfunctions, the inducer motor turns on and then after 30’ish seconds I’m expecting the igniter to begin glowing before opening the gas valve but this doesn’t occur. Then it repeats this same cycle a couple of times. During the start and restart processes I hear a bunch of clicking from the control board relays. I’ve check the pressure switch which does have continuity when I suck on the tube and then when I stop sucking, the multi-meter reads OL. I checked all four flame roll-out switches to see if they were tripped and they were not. I also checked each for continuity and they checked out. There is no flashing code when it malfunctions on the control board. Additionally, when it occurred the first time, of course it didn’t act up so the tech cleaned the flame sensor which didn’t work as evidenced by the furnace continually not turning on. As a result (and before I started to get smart on this furnace), I bought a new flame detector switch which I now realize could not have been the issue. Since I can see the ignitor glowing and the fact that it lights the gas after it rests (and I also checked the ohm reading which if I recall read .089, I assume that is not the problem. I’m guessing that the only thing that could be wrong is the control board. Oh, and I religiously change the filter every month and even put another new one in after only two weeks of service which didn’t help the problem. One other point that my wife noticed today was that the furnace turned on and once it reached the set temp, it continued to blow, but then was blowing cold air.

    Thank you for your help sir.

    Best regards,

    Terry L. Robinson

    1. Hi Terry, Yes after all of the tests and troubleshooting that you performed it sounds like the control board is the problem. I would like to suggest that you make sure all wire connections, ground wire connections, Molex plastic connections are good and tight. I would like to suggest that you make sure that the hole where your pressure switch attaches to the draft inducer is not obstructed. You might want to take a 1/16″ drill bit and make sure the hole in the draft inducer is open. I would suggest making sure the pressure switch is closed when the furnace draft inducer is on and when you are having the problem with the furnace not igniting. We have a really good furnace pressure switch troubleshooting page with two good YouTube videos on the following page: https://arnoldservice.com/how-to-test-and-troubleshoot-gas-furnace-pressure-switches/ It is not advised to suck on a pressure switch because it can damage the pressure switch. Since you said that your wife noticed that the furnace continued to blow cold air after the thermostat was satisfied…this is a sign that the furnace may have gone off on high limit. The control board thinks the furnace has over-heated so it keeps the furnace blower running to cool the furnace down. I would like to suggest that you make sure that the furnace is not going off on limit. The furnace control board blink code should tell you what the problem is. I hope you can easily find and fix the problem soon. Steve

      1. Nice and thank you so much sir! I will test the items you recommended and check all connections to ensure they are tight and that the ground is also doing its job.

        I cannot thank you enough for your thoughts and advice. It is greatly appreciated. I found that young man who produces Word of Advice Youtube to be very helpful. I watched many of his videos this last weekend before testing various components.

        Have a wonderful weekend and I will follow-up here on the additional test and hope to report soon that the problem was resolved.

        Warmest regards,

        Terry L. Robnson

      2. Hi Terry! Thank you so very much for your kind words! Yes, please let us know when you find and fix the problem. I hope you have a great day and weekend! Steve

  3. My furnace hace been malfunctioning. It starts as if it’s heating to then turns off before it begins blowing warm air. The thermostat will say 64° when I have it set at 68. This is just an example. It does this periodically. Hopefully you understand what I just explained v

    1. Hi Alicua! This sounds like you might have a bad blower motor or a fan relay that is on the control board that has gone out. I would like to suggest that you turn your thermostat to “Fan ON” and see if the blower motor will run. If not then you might need a new blower motor or control board. You would need to test with a voltmeter to see where the problem is. If the blower motor is bad the furnace will heat up momentarily then the furnace’s limit will cut the gas off so the furnace will not over-heat. You would need to test the limit with a voltmeter to see if the furnace is going off on high limit. 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. Steve

      1. Thinking it’s the draft inducer motor. Could that be the problem?

      2. Hi Alicia! Yes, the draft inducer motor could be the problem. You would need to see if the draft inducer is getting 120 volts AC when the inducer turns off. If the inducer is getting 120 volts and not running then you will need a new draft inducer. If the draft inducer is not getting 120 volts then you could have a control board problem. If your control board has a flash code try to read the code. Most of the time the control board will tell you what the problem is. The flash code key is usually glued to the furnace’s blower door. I hope you can easily find and fix the problem. Steve

      3. Diagnostic code flashes once which states ignition failure. What do I need to do to fix that?

      4. Hi Alicia! This usually means that the furnace has failed to ignite the burners. No flame. This could be caused by an open safety control like a pressure switch, limit or rollout switch. It also could be caused by a bad control board or a burnt-out ignitor. We have a post about this problem with videos on the following page: https://arnoldservice.com/problem-furnace-will-not-ignite-ignitor-will-not-glow/ If you would like for me to look up parts then please send your furnace’s model number to our email address: arnoldservice@gmail.com and I will be glad to look up parts for you or answer any other questions you have. I hope you can easily find and fix the problem. Steve

      5. All my room in my 28×72 double wide are blowing but the last room is hot why is that

      6. Hi Chance! Maybe a duct has come apart or your blower is not strong enough for your home. We have a post about this problem where I try to explain on the following page: https://arnoldservice.com/why-is-my-new-heating-and-air-conditioning-system-not-heating-and-cooling-my-house-properly/ I hope that you can find and fix the problem soon! Steve

  4. Hello, I just found your website and really appreciate reading all of your advice here. Thank you for being such a great resource for this community.
    I have a York Coleman with a 364810 VARIDIGM Control Board (SCD-1097). On a call for heat, the board displays the correct 2 green flashes. The inductor motor runs, the pressure switch closes right away until the cycle stops – as it should – and stays closed, but the pressure switch phase fails and the board displays two red flashes indicating the pressure switch is stuck closed. So, the furnace will not even ignite. I’ve tested this pressure switch cycle thoroughly with the voltage and the ohms method with a multi-meter. Even so I’ve taken apart and ensured every part of the ducting system and the vacuum system is clear and working.
    I’ve ohmed every wire throughout the system to make sure there are no breaks or shorts. I’ve checked that the proper voltage is coming into the board. I’ve checked that the correct voltage is flowing through the board controls – including the fact that the pressure switch circuit starts on zero volts and within 2 seconds of the induction fan coming on goes to 26 volts and stays at that voltage for about 2 minutes until the 2 red lights flash.
    I called an HVAC serviceman out but he found the same issue and said the control board needs to be replaced. However, I have tried two other used boards and they all test identically with the same errors.
    Is there something external to the board I am overlooking? I’ve replaced the igniter, the pressure switch (with the exact OEM model switch), the flame sensor, and the temperature limit switch (all OEM exact parts).
    Or could it be that all these boards fail and that I’ve got three bad boards?
    The furnace has run only a couple days this Fall before failing. Initially it was a ground/polarity fault (9 flashes) but that seems to have gone away for now and been replaced with this pressure switch error code.
    I saw that you recommended to someone that they check voltage from W to C at the control board. I haven’t done that yet. Would that help me? I have spent well over 10 hours on this and am running out of ideas.
    Thank you! Glenn

    1. Hi Glenn! Thank you so very much for all your kind words and for asking this question. Yes, I would like to suggest that you make sure that you are getting a constant 24 to 28 volts AC between the W and C (com) thermostat wires when your thermostat is calling for the heat to be on. Usually well the control board shows the stuck closed flash code then this means as you probably know that the pressure switch did not open up after the draft inducer turned off. Pressure switches that stick closed could be caused by having water or and obstruction in the pressure switch tubing or where the pressure switch tubing connects to the draft inducer. Make sure the hole where the pressure switch connects is open. I have had to use a 1/16″ drill bit to open the holes up before because they become clogged so bad. If you have a condensing furnace that produces water when it is on then make sure that the condensate drain on the furnace is open. Disconnect the new pressure switch that you installed and use a wet vac to suck out the condensate drain line and draft inducer hole. It is unlikely the pressure switch is bad since you replace it and many other parts. You can see if the pressure switch is actually bad or stuck closed by disconnecting the two wires that go into the pressure switch with the furnace power switch on so that the control board senses the pressure switch is open. Turn the power back on to the furnace and turn the heat up so the draft inducer starts. Join by hand the two wires that go to the pressure switch so you are simulating the pressure switch closing. Your furnace is working properly should go through one cycle until either the thermostat is satisfied or you disconnect the two pressure switch wires. The pressure switch has to open after each heating cycle. If it does not open then you will get the closed pressure switch error flash code. I would suggest making sure that if you have any Molex plastic connectors that you unplug the connector and plug it back in. I have seen the wires inside the connector arch and not make good contacts. If you have batteries in your thermostat make sure the batteries are good. Yes, if you are purchasing used control boards then all three control board could be bad. If you want me to look up parts please send me your furnace’s model number to our email address: arnoldservice@gmail.com Let me know if you have any questions. I hope you can easily find and fix the problem soon! Steve

  5. Hi Steve. Great site….very informative. I have an older propane furnace that is giving me intermittent grief. The sequence of operation is slightly out of order. Call for heat starts the inducer motor and the pressure switch closes. The main blower will then come on, followed by the ignitor, then gas. Once the desired heat is reached everything shuts off. The next cycle is the same but the ignitor may or may not turn on. The control board will indicate two flashes – pressure switch stuck closed. I can correct this by removing the hose from the pressure switch to the inducer and then replacing it again. The ignitor will then fire up and the heat will come on. The out-of-order cycle is new. It only started happening when I turned the heat on this fall. The furnace produces heat about 50% of the time. The other 50% I have to pull the air hose off the inducer. Any thoughts?

    1. Hi Andy! Thank you so very much for your kind words! Sounds like you might need a new pressure switch or you have a problem with the drain of venting on your furnace that is causing the pressure switch to stay closed. I would like to suggest that you make sure that your condensate drain on your furnace is open by taking the pressure switch tubing off so the pressure switch does not get damaged and use a wet vac to clean the condensate drain, draft inducer and secondary heat exchanger out. Make sure that your vent is not obstructed or blocked with anything. If cleaning the condensate line and draft inducer does not work then I would suggest a new pressure switch. If you would like for me to look up parts or the pressure switch then please send me your furnace’s model number to our email address arnoldservice@gmail.com. I hope you can find and fix the problem soon! Steve

  6. Hi Steve, just stumbled on your informative site. I have a Goodman furnace that turns on then after 40-50 seconds turns off for a second or two, then turn back on and stays on till set temperature is reached with no issues.

    1. Hi Moh! This could be hard on to troubleshoot because you almost have to be there troubleshooting with a voltmeter when the furnace turns off. The first thing that I would check would be the thermostat and I would suggest testing with a voltmeter set to volts AC. Make sure you are getting a constant 24 to 28 volts between the W and C (common) terminals on your control board. You will probably need to tape your blower door safety switch down so you can get to the control board and do this testing. If the voltage is dropping out when the furnace stops then you have a thermostat problem. Make sure that if your thermostat uses batteries that the batteries are in good condition. Next, I would test the pressure switch by placing one voltmeter lead on each terminal and one meter lead to a good ground (bare metal on the furnace). You should get 24 to 28 volts from each pressure switch terminal to ground when the draft inducer is on. If not then you have a bad pressure switch or a problem that is not allowing the pressure switch to close like a partially stopped up vent or heat exchanger. I would test the limit next and make sure the limit is staying closed. If all these check out OK then I would suspect that you have a control board problem where one of the relays on the control board are not making good contact all the time. I hope you can easily find and fix the problem. Steve

  7. ok I have a heil 90 percent furnace following the chart inducer comes on ignitor glows bright yellow pilot lights 7 or more seconds burners do not light going down chart I get to check volts at c and w on board nothing say faulty wires therm or faulty thermostat so I but on new thermostat it will do nothing put old one back on same stuff I did not change the wires any thoughts thank you for your time have a bless day

    1. Hi Rick! Thank you so very much for your blessings! Sorry that it took me so long to respond. We have been packing orders and going to get parts. If you are not getting 24 volts between the C and W connections when your thermostat is calling for the heat to be on, then this is as you probably know a sign that you have a low voltage problem. I would like to suggest that you test between R and C. R and C are the connections that come directly from your low voltage transformer. If you are not getting 24 volts there then you either you are not getting high voltage to the primary of the low voltage transformer or the secondary of the transformer is out and you might need a new transformer. I would also check to make sure that the fuse (if equipped) on the control board is not blown. You can test to see if the thermostat is the problem by turning off the power to the furnace then tying the Red and White thermostat wires together with a wire nut. Turn the power back on and the heat should come on. If it does not then you have another low voltage problem. 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. Steve

  8. My gas furnace runs great until December. Then it tries to start up and shuts off, tries again 2 more times and shuts off each time. Then the furnace shuts down. I go down with an old tooth brush and clean the heat sensor with the switch off. Turn the switch back on and it fires. It can do this periodically or 3 times in a weekend. Sometimes I just turn the switch off, wait a few, turn it back on and it will fire. My worry is that one day this will happen after I’ve left for work. The heat would then be off all day until I got home. Please help.

    1. Hi Gail! Sounds like you are having furnace flame sensing problems. We have a flame sensor troubleshooting post of the following page: https://arnoldservice.com/problem-your-furnace-main-burner-ignites-the-main-burner-stays-on-for-8-to-10-seconds-then-shuts-right-back-off/ The Youtube video near the bottom of the page is good for troubleshooting flame sensor problems. If replacing the flame sensor, and checking all wire connections to make sure they are good and tight does not work then you might need a new control board to fix the problem. 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. Steve

  9. What is the best way to learn about electrical on a furnace and the air conditioner

    1. The best way if you wanted to go into business would be to go to a technical school that is NATE certified. If you want to learn as a DIY person then I would recommend following the Youtube videos on the AC Service tech channel. Here is a link to that Youtube channel: https://www.youtube.com/watch?reload=9&v=hJqRfjDE4d0 He has some great Youtube videos on air conditioning and furnace repair. I hope you have a great day! Steve

      1. Hello, have baseboard gas heat broiler, the pump went and was changed, ever since the heat will not kick on unless I bump thermostat up to 75, and when it does kick on it works really well, and need to shut it off after about half hour, becomes way too hot in house. We also changed thermostat, no luck, any suggestions, never had any issues at all until we had to change pump.

      2. Hi! I am sorry, but I really do not know much about boiler heating. The only thing that I can think of that might be causing this problem with the thermostat would be maybe a loose low voltage wire connection or the new boiler pump might be drawing too many amps for the thermostat to handle. In other words, the new pump could be throwing the heat anticipation/anticipator off on the thermostat. Most of the time a new digital thermostat will fix this problem. If you are interested here is a link to a good Youtube video on setting thermostat anticipation: https://www.youtube.com/watch?reload=9&v=clud3BBiQV8 Sorry that I can not be of more help. I hope you can find and fix the problem soon! Steve

  10. Steve, I have a Nordyne FG6RC-080C-12B (happy to send you a pic). Just replaced (within last week), pressure switch, limit switch, control board, flame sensor, flame igniter and a new Honeywell Thermostat.

    When i call for heat the furnace kicks on; steady red LED (power), then yellow LED (flame), burner box heats up, runs for several minutes, shuts down, 2 red flashes (pressure switch), then one red flash (limit circuit), then furnace cycles and operates fine again – does this until desired heat is reached.

    I notice gurgling (suction of water) from a drain tube that is connected to the bottom of the header box (black box behind blower motor) – which i think should be draining into the trap – not sucking.

    Any thoughts?
    David

    1. Hi David! I would suggest that you check with a volt meter to see if the limit is opening up or if the pressure switch is opening up as the flash code suggests. I would suggest that you disconnect the pressure switch sensing tube and use a wet vac on the furnace’s condensate drain line to make sure that the furnace’s drain is open. If you can pour some water down through the furnace’s drain then this is a good test to see if the drain is open. We have a pressure switch troubleshooting page that I hope will help you out: https://arnoldservice.com/how-to-test-and-troubleshoot-gas-furnace-pressure-switches/ and we have a post about troubleshooting a limit switch on the following page: https://arnoldservice.com/problem-furnace-comes-doesnt-run-long-shuts-off-will-work-thermostat-turned-way/ We have lots of furnace troubleshooting advice on the following page: https://arnoldservice.com/gas-electric-furnace-troubleshooting-simplified-2/ 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. Steve

  11. Hi Arnold , I got York furnace in the attic and goes out on auxiliary limit switch after it ran for couple hours . I had to Manuelly reset every time . This switch is located in blower compartment . Please help

    1. Hi Pete! More than likely if you have a down-flow furnace either you have a blower motor that is going off before getting all the heat out of the heat exchanger or you have some kind of restriction in the blower motor or heat exchanger that will not allow the heat to be expelled out by the blower. The blower motor might not be running long enough on a downflow furnace to get the heat out of the heat exchanger, thus the heat comes up and causes the limit to go off. If you have a fan control where you can adjust it, then try to adjust the control so the fan will run longer to get the heat out of the heat exchanger. I would also suggest that you make sure the blower motor capacitor is in good condition. I hope you can easily find and fix the problem.

  12. I enjoyed reading this. Thank you.
    I have an York Stellar that was put in in 1989. This fall it started acting up. It runs fine for a few hours then fails to fire on heat call. Just replaced the flame sensor.( Cheap ) with no change in its bad function. When it does work it lights-fires and completes a cycle. The code seemed to be show red flash when it fails. I checked the incoming and outgoing flue and did remove a VERY old filter that was sucked into the furnace at some point but was stuck before hindering the blower squilcage.. It is frustrating but I did find YORK parts are easily available or in stock.. Just no clue what needs replacing. FYI The burners are a beautiful blue with no yellow when lit. HELP… small town HVAC dude is worthless.( Third visit from him and no help at all.)

    1. Hi Davis! I would suggest that you try and read the flash code on the control board to see what the control is showing that the problem is. The flash code interpretation should be located on the furnace blower door or somewhere on the furnace. After you find out what the board is saying the problem is then you can concentrate on correcting the problem with new parts or cleaning parts. I would suggest making sure all wire connections are tight for a start. 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. Steve

  13. I have an older carrier gas furnace. The inducer starts and the pilot comes on, but no flame. I ran a115v from terminal R to terminal 5 and the flame started and the furnace ran perfectly fine. Does this mean that the gas valve is good, and there is another problem?

    1. Hi Tammy! Many of the gas valves have two solenoids inside the gas valve. The PV (pilot valve solenoid) and the MV (Main gas valve solenoid) Yes, since the PV (pilot valve) on the gas valve is opening, your pilot is lighting and the MV (Main Valve) gas valve is coming on with you hot wiring it, then this would mean that the gas valve is good. Sounds like you might have a problem with the control board or other electrical problem that is not allowing 24 volts to get to the main gas valve. I would suggest making sure all safety controls are closed like the pressure switch, limit and rollout switches. I hope you can easily find and fix the problem. Steve

  14. This is exactly what I needed, thank you.

  15. Hi, my natural gas furnace is about 3 years old. I have now noticed that the high and lower speeds no longer work. It just blows on high, no matter what speed you put it on. It also blows cool air for about a min, than shuts off, then blows warm air. Please advise

    1. I am sorry, but I do not know for sure what the problem with your furnace could be because I do not know how the installers hooked up the thermostat on your furnace. Some two speed furnaces use a 2-stage thermostat to switch between high and low speeds so this could be a thermostat problem. Some installers use a regular singe stage thermostat and allow the control board to control the time the furnace runs in low and high speeds. So you could have a control board problem. I have our Bryant 90I home furnace’s control board set to have the speeds, low-fire and high-fire determined by the control board. If our furnace runs longer than 10 minutes on low speed the high speed automatically kicks in. The control board determines the number and length of the heating cycles and determines whether the furnace needs to be in low or high heat. If the thermostat or another control is short-cycling (not allowing long run times) then your furnace’s control board may have determined it needs to be in high heat because of the short cycles. The thermostat or another control could be causing this problem. I would suggest starting with the easy solutions first by checking all wire connections and plug-in connections to make sure they are good and tight. I would suggest testing your limit to make sure it is staying closed and not short-cycling the furnace. If everything checks out OK then you might need to call a service tech because these new 2-speed furnaces can be complex. I hope you can easily find and fix the problem. Steve

      1. Thankyou Steve !

      2. It worked good untill I moved the thermostat. I had to add an extension on to the wires and like a dumby I didn’t turn the power off, so one of the thermostat wires touched another bare wire. So now I believe I fried the mother board. It is a Miller and was professionally installed

      3. Hi! The only way you could have hurt the control board would be if there is a common wire on the thermostat (five or more wires going to the thermostat). If you touched one of the other wires with the common wire it could burn up a component or cause the fuse to blow on the control board. If this happened I hope your control board has a fuse to protect the low voltage components. I hope you can get if fixed. Merry Christmas! Steve

  16. I have a train XL 80 furnace that is 23 years old, everything works fine except the power vent pipe seems excessively hot. (200° at 12 inches above the furnace). What should the temperature range be on the power vent pipe during heating cycle at the top of the furnace.? Too hot could be a cracked burner, but the blower and the Hi/low vacuum switches work perfectly. If there was a cracked burner would it not be getting enough vacuum to operate properly? The power vent pipe is 14 feet straight out with one elbow and is not blocked.

    1. Hi Doug! I would think that on an 80% furnace the vent pipe should be pretty hot. Hot enough to burn you. I do not know what the normal temperature of the vent pipe should be. The furnace has safety rollout switches, limit and pressure switch that should protect you from a dangerous situation. If the heat exchanger is leaking it should trip one of the roll-out switches and yes affect the pressure switch where it would not work properly. Maybe you have the gas input set too high. If you are getting real hot air out of your registers then you might think about adjusting the gas pressure down a bit or placing your furnace blower motor on a higher speed. This can be dangerous in doing this because if adjusted too low it can cause delayed ignition which would cause a slight explosion. I have seen furnace doors blown off from delayed ignition. You might want to get a tech to check it out if you are concerned. Sorry that I can not be much help. Steve

  17. Hi

    I have a Lennox High Eff. furnace that is approx 22 years old. Problem is that the heat shuts off and blows cold air. I have had a tech try to fix for the last two weeks and each time they try a new part and it works for different lengths of time, then the problem comes back. So far they have replace the t-start, control board an air diaphram, and now the gas valve as recent as yesterday. Upon installing it worked when they left yesterday, but at some point in the night it stopped heating again and blew cold air all night. Not sure what the problem might be. Also, I am so far into this with cost to repair and it is still not working. Any information would be a help. My e-mail is lparker@mcmunnandyates.com

    Thanks Larry

    1. Hi Mr. Parker!
      Most of the time when a furnace shuts off and blows cold air it is because the furnace has over-heated. The control board thinks the furnace is too hot so it turns the blower on constantly to cool the furnace down. I would like to suggest that the next time that this happens that you check all the roll-out switches and limit to make sure they are not open. We have a post about this problem on the following page: https://arnoldservice.com/problem-your-fan-on-your-furnace-runs-all-the-time-and-will-not-shut-off/ Other problems that can cause a limit to open up would be poor air flow like a dirty filter, a stopped up evaporator coil a slow or dirty blower wheel (a weak blower motor capacitor) You might need a new blower motor capacitor if the motor is slow. I hope you can easily find and fix the problem. Steve

  18. Hi Steve, I have a goodman GMVC80704BXAA gas furnace. During start up the burner igniter will not power up. If I short out both of the pressure switches after the inducer motor starts up, then the igniter will power up and the burner will fire up. It runs normally until the next power up cycle, but again it will not fire, unless I short out the switches. If I apply vacuum to either switch, I hear a clicking sounds, but I get no continuity. Does this indicate both pressure switches are bad?

    1. Hi Bob! This sounds like you either have a pressure switch problem or a induced draft problem. We discuss this on the following page: https://arnoldservice.com/how-to-test-and-troubleshoot-gas-furnace-pressure-switches/ Most of the time on a two stage furnace like you have the 1st stage pressure switch closes first, but sometimes the draft inducer builds up to high speed (high heat), both pressure switches close, until the flame ignites and then kicks back to low stage. I am sorry, but I do not know the sequence of operation on your furnace as to whether both pressures switches close at first or just one. Our Goodman parts program shows that your furnace uses pressure switch part number: 0130F00049. I am sorry we do not stock this pressure switch. Please follow the advice in the post I sent above. If none of these are the problem then I would suggest a new pressure switch. If you want me to send you a complete parts list for your furnace please email me at arnoldservice@gmail.com and I will be glad to try and help. I hope you can easily find and fix the problem. Steve

      1. Hello
        My furnace does the opposite, it ignites no prblm, aftr it ignites the flame runs 3 or 4 secs an shuts off an the cycle strts over again, it will do this 4ever. Anothr prblm is when it does stay lit it runs constant so then overheats an blows cold air… An thats when the prblm strts that i told u 1st… I was thinkin of usin the trbleshootin flowchart…

  19. Burner comes on, blower comes on. Burner continues fine while the blower comes on and off repeatedly. The cycle continues until the thermostat is satisfied.
    High limit switch?

    Thanks in advance!

    1. Hi Steve! This sounds like this could either be a blower motor problem or a control board problem. I would like to suggest that you test with a volt meter set to “Volts AC” the connections that come out of the control board that go to the blower motor. You would test on terminals marked “Heat” and one of the white neutral connections. You will probably need to tape the furnace blower door safety switch closed so you can do testing with the power on. Set the thermostat so the heat is on and notice if the power goes off and on (120 volts to 0 volts) to the blower motor or if the blower motor gets constant 120 volts and still goes off and on. If the voltage drops out of the control board then you probably have a control board problem. If the voltage stays steady and the motor still goes off and on then you probably have a blower motor problem. I would suggest making sure the blower motor capacitor is in good shape. I do not believe it would be a limit switch because if a limit is open the blower usually runs all the time. If you have an old fan limit control like the Honeywell we sell on the following page: https://arnoldservice.com/product-category/fan-blower-controls/honeywell-fan-and-limit-controls/ then yes, it might be a fan limit control that is giving you this problem. Most of the furnaces now have a control board that controls the fan. I hope you can easily find and fix the problem. Steve

  20. My furnace will call for heat, turn on the inducer motor and then will not fire the burners. I have tested the voltage from the gas control valve to the pressure switch and it reads 23 volts. Out of the pressure switch it reads around 15. I have tried to jumper the wires off the pressure switch and had the same results. I am getting 27.8 volts into the gas control valve, but for whatever reason, I don’t ever get 24 volts to or from the pressure switch. I even replaced the pressure switch and got the same results. Any help is greatly appreciated.

    1. Hi Eric! Sorry to hear that you are having trouble with your furnace. Since you are not getting 24 volts out of the pressure switch I would suggest that you make sure that your low voltage transformer is grounded well to the frame of the furnace. Sometime a loose grounding screw can cause low voltage. Is your ignitor glowing because on most furnaces if the pressure switch does not close the ignitor will not glow. Another thing to look at would be any connections from the pressure switch to the control board. Make sure all connections are tight and not burnt. Sometimes a molex connector will be burnt or loose and cause the furnace not to work right. I would suggest testing with a volt meter set to “volts AC” between the R (red) and C (com) connections on your control board. You should be getting 24 to 28 volts AC during furnace start up. If you aren’t then you might need a new transformer or have a loose ground wire. I hope you can easily find and fix the problem. Steve

  21. when the flame kicks on to heat up before the blower comes on, why does it sound like there is air in it (only way I know how to describe it) then it goes back to normal.

    1. Hi Judy! The only thing that I can think of that might cause this would be, maybe the burners need to be cleaned or there is too much gas pressure going to your burners. You might have a tech check to make sure the gas pressure isn’t too high. Sorry, but that is all that I can think of. Hope you get it fixed soon and inexpensively. Steve

  22. Ignitor is new, voltage to ignitor is 120v, flame sensor id clean. Blower runs for approx 10 seconds, no glow at ignitor, then shuts down. Fuse ok at the control board. Thermostat has 24 volts in and out. What else should I check? THANK YOU.

    1. Hi Nick! You say the voltage to the ignitor is 120 volts and the ignitor is not glowing? Please check the voltage coming out of the control board with the ignitor plugged in if possible. Sometimes the voltage going to the ignitor will show 110 volts without the ignitor plugged in and when you apply the load of the ignitor the voltage will drop to zero showing that you have a bad relay on the control board. If you have 120 volts coming out of the control board to the ignitor with the ignitor plugged in, then you must have a bad ignitor because it should be glowing with the voltage applied. If the ignitor does not glow at all then I would suggest checking to make sure all safety controls are closed like the limit switch, roll out switches and pressure switch. The first things that a control board checks for in the ignition sequence is to check and make sure the draft inducer is running and the pressure switch is closed. If the limit and rollouts are closed the control board sends 110-125 volts to the ignitor. If any of the safety control are open then the control board will shut down the furnace. I hope you can easily find and fix the problem. Steve

  23. Installed new blower with add on capictor,old motor did not have an external one.now furnace warms up shuts off after a few min and blower comes on.it cycles like this till thermostat is satisfied.

    1. Hi Joe! Most of the time a blower motor with a capacitor is better and more efficient. Sounds like you might need to adjust your fan limit control if you have this type of fan limit. If you have an older furnace most of them have adjustable fan controls like the Honeywell controls we have on the following page: https://arnoldservice.com/product-category/fan-blower-controls/honeywell-fan-and-limit-controls/ The newer furnaces control the timed off/ON of the blower motor through the control board. I hope you have not, but you might have the blower motor wired incorrectly. Please make sure you follow the wiring diagram on the motor label. I hope you can easily get this problem corrected. Steve

  24. Furnace runs for an awful long time after flame goes out to cool the heat exchanger is there a reason why

    1. Hi Tom! High Efficiency furnaces run long times so they do not waste any heat in the heat exchanger. This is part of the high efficiency. Most furnace have dip switches on the control board that will allow you to adjust the time the furnace stays on after the gas shuts off. I would suggest that you look at the label on your furnace or the owner’s manual and see if the furnace has any dip switches that you can set to cut the run time down. I hope you have a great day! Steve

  25. My furnace runs for 5 min. Then shuts off for 2 min, then repeated the same cycle
    The flame sensor was cleaned by the furnace employee.
    Still have a problem.Do flame sensors go bad?

    1. Hi! This does not sound like a flame sensor problem because the furnace is running for 5 minutes. If the burners were cutting off in 8 to 10 seconds then I would think it was a flame sensor problem. I would suggest that you check your limit to see if the furnace is going off on limit, then cooling down, then coming back on. If your furnace is going off on limit then you might have a dirty filter, blower wheel or a stopped up evaporator coil. A slow blower motor (maybe a weak capacitor) can cause an over-heating condition. I would also check between W and C (com) to make sure the thermostat is telling the furnace to stay on when it goes off. You should get a constant 24 to 28 volts AC between the W and C terminals on your control board when the thermostat is calling for heat. Best of luck in finding and fixing the problem. Steve

Leave a Reply

Your email address will not be published. Required fields are marked *