Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

80% Gas Furnace Not Infighting - No voltage to Gas Valve - Soft Lockout every time

Michael001 Member Posts: 4
edited November 2020 in Gas Heating
My furnace burner isn't producing flame. It was working when it last ran early this year. The only blink code I can get out of it is for "Soft Lockout."

80% Rheem / WeatherKing: 92-101654-01-01
IFC board: 1012-83-9257a

Here's what happens at a high level when I turn on the furnace with thermostat requesting heat.
  1. Purge for a couple of minutes
  2. Induced Draft Motor runs
  3. HSI glows for about 10 seconds
  4. HSI stops glowing without infighting
  5. IFC blinks code 1 (soft lockout)

Here's what I've already checked.
  1. The thermostat is requesting heat.
  2. The pressure switch closes and has about 27 volts across it after IDM starts. (Sorry, I was wrong about this. The 27 volts is there after the IDM stops, and briefly before it starts. It is 0 when the IDM is running.)
  3. The gas valve does not gets any voltage to it.
  4. The wires connecting the control board and and gas valve have continuity.
  5. The pressure switch remains closed once initially closed during startup (actually, it opens again once the Soft Lockout occurs and IDM stops), but is open before IDM runs (i.e. does not appear to be stuck closed or open).
A troubleshooting guide was inside the door. I highlighted the path I took through it in green here.

I am not sure what specifically to check for the "Ensure 24V between appropriate pins on connector of IFC." part. I don't know which pins are the appropriate pins in this context. I do get 24 volts between C and W, but I don't think that's what this is taking about.

I have 2 of the same furnace (one in attic, one in basement). The working one in the attic gets 24 volts to the gas valve shortly after the HSI starts glowing. Everything before that seems the same as the non-working one.

Any ideas on what else to check?


    HVACNUT Member Posts: 5,255
    Something in the pressure switch series is open. The pressure switch, a limit. If the pressure switch closes, you should read 0 on your meter. Check continuity across the pressure switch and other safeties. With the wires off.
  • ratio
    ratio Member Posts: 3,507
    @HVACNUT pinned it, I think. Check the pressure switch again. If you're measuring the voltage across the two terminals of the pressure switch, you're measuring the voltage drop across it. Closed, you should be seeing something like zeroish volts (contact resistance, mainly).
  • Michael001
    Michael001 Member Posts: 4

    I'll take a closer look at the pressure switch and other limits. I was expecting it to show a code 2, 3, or 4 for that based on that chart inside my furnace door, so I might not have checked that as closely as I should have.

    I'll check continuity when it is off and resistance during startup. I already checked continuity when it was off, and believe it had continuity in both the working and non-working furnace when it was off, but I'll double check that when I can (at work right now).

    I also may have been misleading / over-simplified with "The pressure switch closes and has about 27 volts across it after IDM starts". I should clarify it doesn't constantly read 27 volts until after startup is done. I remember it dipped to 0 at some point during the startup (I think as soon as the IDM started) in both the working and non-working one. I'll have to double check, but I believe it went:
    Power off to whole furnace: 0v
    Power on: 27v
    IDM Running: 0v
    Lockout (and motor off): 27v

    Anyway, I'll double check. Thanks for the help so far.
  • Jamie Hall
    Jamie Hall Member Posts: 21,536
    If it's showing 27 volts across the terminals, the switch is open -- and, unless I'm mistaken, that switch must be closed for the system to run.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    Make sure there is no blockage in the pressure switch tubing causing intermittent operation.
    EBEBRATT-Ed Member Posts: 13,799

    With most furnaces with no call for heat the pressure switch contacts must be open......this proves to the board that the switch is not defective or jumpered.

    When the inducer motor starts, the pressure switch must close within a few seconds if it doesn't the hot surface ignitor will not turn on.

    For testing purposes only, when the inducer get power jumper the pressure switch and then you should get the HSI to glow and the gas valve should power up. Check for voltage at the gas valve. You only have a few seconds to check this before trial for ignition ends.

    If you have power you could have a bad gas valve or no gas to the furnace.

    If you have no power refer to the wiring diagram and follow the gas valve wires back to the board

    MAKE SURE to unjumper the pressure switch. And blow out the pressure switch tubing.

    If the pressure switch is erratic check the flue for blockage, birds nest etc
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,074
    edited November 2020
    I like @EBEBRATT-Ed's assessment and procedure for your situation. Whenever I find what I believe is the problem (in your case the pressure switch), I look at what it is supposed to do. then determine if the part is defective or if the part is operating properly and doing its job. Your fan switch open may be due to a blockage. that is why it is there, to make sure the burner won't rin if the exhaust is blocked. Remove the blockage then the problem is solved.

    This goes back to a question I would ask the technicians that worked for me: After you change the part and it does the same thing, what will you check next? Many times the next thing is less expensive than replacing the part (that was working just fine) In your case there may be a blockage in the vent pipe or there may be a blockage in the tubing that connects the pressure switch to the vent system.
    Just using the switch as an example
    That condition would give you a different 2 blink error code.

    Sometimes the electrical connections on the different components get some corrosion buildup. This is easily fixed by carefully removing the wire connector from the connection and just immediately replacing the wire. To do this to the 9 pin plug on the control board, squeeze the clip that secures the Molex plug on to the IFC board and carefully rock it while pulling the plug. Once removed, put the plug back in. Check the wires that connect the other controls to the board and to the controls themselves. Don't be a muscle man because some of the controls us small rivets to connect the spade connectors to the control. if the connection is very tight and corroded onto the spade tab, it may break off the control.

    Sometimes just a little movement is enough to re-establish the connection.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Michael001
    Michael001 Member Posts: 4
    I double checked the pressure sensor readings. The clarification I added earlier (0, 27v, 0, 27v) seems to check out, but I think a video can show it better than words:


    So, unless I'm misinterpreting this, the switch is closed while the IDM is running and is only open briefly before the IDM spins up, and then constantly after the Soft Lockout happens and the IDM stops.

    Does that not mean the pressure sensor is working as intended?

    The working furnace is the same, except it gets 24 volts to the gas valve after the pressure switch closes and it lights, so doesn't lockout.

    This is the working furnace (different furnace in my attic, but same model)
  • Jamie Hall
    Jamie Hall Member Posts: 21,536
    An attempt to clarify here, since the exact timing is critical: the pressure switch opens and then the fan stops, or the fan stops and then the pressure switch opens? Which, exactly, it it?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Michael001
    Michael001 Member Posts: 4
    I think the fan stops, then pressure switch opens. I hear and see it slow down before it closes (voltage goes to 27). It is harder to see in the video, but you can hear it slow down at around 44s and the voltage isn't there until around 46s
  • Jamie Hall
    Jamie Hall Member Posts: 21,536
    OK. That's what it should be doing -- which means that it isn't likely to be the source of the problem.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 10,791
    Is your IDF motor dropping out? Amp clamp on the leads would tell you if it falls out. Usually the IDF is called on by the call for heat and the pressure switch, HSI or gas valve will not shut it down.
    The control board will shut it down after post purge when the heating call is over.