Troubleshoot the pressure switch on the 80% AFUE gas furnace

A description of the steps to troubleshooting pressure switch problems.
This video is part of the heating and cooling series of training videos made to accompany my websites: and to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

24 Replies to “Troubleshoot the pressure switch on the 80% AFUE gas furnace”

  1. If you put the hose on the right port of manometer would you see the negative sign? Or you get same value with no negative sign. Thanks

  2. Have you ever seen an intermittent pressure switch that would only fail on start-up? It fails to start the first try sometimes not always though…usually the second time always… very few ever does it take 3 times. Once it starts and begins running it nevers shuts off until the thermostat is satisfied and cycles completely.
    This is a new Lennox 80% furnace replaced 12 -31-2020 and has had this issue since day 1.
    I noticed it took twice to start the day the techs installed and had left but just thought it was a fluke of air in gas line or something it started fine the second try. Later on i noticed this was something that happened every once in a while so i called and a tech came out and couldn't duplicate a failure so.let it go because it never ever failed to start completely.
    Better half was down stairs other day and noticed it took until the 4th try to start so i called and a senior tech came out and got it to fail for him said he thought pressure switch was causing it and would order and would see results……part should be in soon and will see what happens. Its strange to me in the sense that it only occurs on start up and never again and also that it doesn't fail every time .

  3. Does it not matter which side the hose is on? I noticed you have it on the pos side. I thought pressure switches were checked on the neg side?

  4. Coming from Commercial and Industrial background these videos are great to learn the troubleshooting side of things.
    Sure helped me understand potential list of issues and fixes.

  5. I have a question. could tell me the ID of the tee you used to connect the manometer?

  6. My Goodman model GDS8703AXBC IS acting up!
    When turned on this is what happens!
    1.When I hear the first click the fan pops on!
    2.When I hear the second click the heating element glows(WHICH was replaced)
    3.When I hear the 3rd click I hear the gas coming out the pipe but it doesn't lite!( until I blow on it)
    4.I have to keep blowing on the flames so the second fan won't blow the pilot out when it comes on!
    If I don't blow on it it goes thru 4 cycles then gives me a code! 7 SHORT BLINKS! I Looked on the units door it says 7 BLINKS( LOW FLAME SIGNAL)!!!! SIGH! DON'T KNOW WHAT TO DO!!!!

  7. Seems that the pressure switch could be checked with ohm meter and just remove the hose at the inducer motor, suck on same hose and look for open close of pressure switch. This could be checked this way if you do not have a manometer.

  8. Could you give me the reference for the tee you in this video or where I can find it at.

  9. You didn't show us how to connect the pressure switch to the manometer and how to connect the meter to the switch

  10. Thank you Mr Gray for all this awesome videos, you are a super Great HVAC Tech, I'm learning a lot with all your good videos,
    Thank you!

  11. So far, you have explained everything in all your videos very clearly. Better than my Heating and A/C instructor. Thank you for explaining certain parts and their functions in the furnace

  12. New install go to start and nothing 3 blinks red light presure switch If I blow on it the pressure switch and I'll fire up when I let go of the tube it stops ?

  13. Quick question about the LP pressure switch. If I wanted to pump down a unit into the liquid receiver (trap the gas between the suction King valve and the liquid receiver King valve) how can I legally stop the LP from tripping? Can I set the differential down to 0? Or set the differential as high as the cut in? E.g 100psi cut in, 100psi cut out?

  14. I've┬áseen excessive resistance across a closed pressure switch… 60 to 80 ohms… It was causing intermittent cycling…

  15. Thanks I had code indicating pressure switch was closed, After watching video I found dead spider and nest in tubing between inducer and pressure valve . You the man

  16. Thanks, it was a water leak right about the switch, which I didn't see it before, but I will check the switch now

  17. Hi GFM, If I disconnect the hose to the burner box, my furnace will fire up. Does this indicate a bad pressure switch?

  18. My Lennox furnace will not come on. When I shake the wires on the pressure switch it comes on. The furnace will work for a couple of days and then the same thing. Does it sound like the switch?

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