Here are my observations on this 8-year-old 95% Goodman furnace: (These are all intermittent occurrences)

  1. Ignition sequence proceeds to igniter on, burners do not light, igniter off. 1a. Ignition sequence proceeds to igniter on, gas valve can be heard and felt opening, burners do not light, igniter off. (if these failed ignition attempts occur 3 times, the system locks out with an E0 error code)
  2. Ignition sequence proceeds to igniter on, burners light but with some blowback or flameout.
  3. Following a lock out, removing the black drain hose from its fitting in the drain pan behind the inducer may allow a few ounces of condensate to flow out, but this does not occur consistently with every lock out. (Residual condensate here likely prevents the inducer from producing sufficient draft to allow the ignition sequence to continue).
  4. Furnace may shut off before reaching the set temperature. Because the thermostat is still calling for heat, the ignition sequence begins again, but under these circumstances, a lock out always seems to occur.
  5. The only error code seen is E0 (lockout due to excessive ignition failures). Note: Ventilation plumbing has recently been redone using a concentric vent system. I am very confident that there is no issue with the exhaust venting. Note: The condensate pump is functioning properly and there is no water leaking out onto the floor. Note: There have been no elevated CO or explosive gas warnings signaled by a new Kidde Nighthawk combination detector next to the furnace.
  • Did you clean or replace the flame sensor ?
    – Traveler
    Jun 5, 2023 at 0:40
  • Natural gas or Propane? I have the same problem with my boiler (propane) and the tech says that Propane burns much dirtier than natural gas and that all his customers that have propane have issues and need annual cleaning / service. Jun 5, 2023 at 4:34
  • Have you tested the (assumed) hot surface ignitor temperature and made sure the vent is clear? I don't see them in your list and they should be validated. @Ruskes this is a 95% so it's probably got a hot surface ignitor not a pilot light.
    – KMJ
    Jun 5, 2023 at 17:03
  • The flame sensor checks out fine.
    – John
    Jun 5, 2023 at 18:12
  • 1
    The furnace condensates drain from the inducer and the heat exchanger into the top of a brick size plastic P-trap and then out from the center of that trap to the condensate pump.
    – John
    Jun 7, 2023 at 16:07

3 Answers 3


You're right to mostly set aside concerns about the flame sensor for now -- when you're standing in front of the furnace and witnessing that the gas really doesn't ignite, you can be sure the flame sensor did not play any role in the problem.

It kind of sounds like you have a fuel delivery problem. Consider verifying the fuel inlet pressure is correct. I believe there's a pressure test port on the outlet side of the valve too but I'm not sure. Also you can apply power, probably 24 volts, directly to the gas valve to test it, though it's hard to do more than sniff and conclude "yep, there's gas coming out." Obviously this will cause a release of fuel gas -- take appropriate precautions, especially since your propane will accumulate on the floor rather than magically dissipate as natural gas would do. You may be able to remove the valve and visually inspect for debris inside it.

You could try igniting the gas yourself. When the HSI begins to glow, bring the flame of a torch or multi-purpose/grill lighter into the area as if it were a pilot light. Watch for weak ignition/blowback/flameout.

It's not alarming that a small amount of condensate might remain in the bottom of the collector box behind the inducer. Literally a few ounces -- 1/8 to 1/4 cup of liquid -- does seem a bit much. If you don't hear it splashing in the inducer though then it's probably not excessive.

Perhaps surprisingly sometimes a furnace will be unreliable when there's too little condensate in the trap! The inducer creates suction, of course, and if the condensate trap isn't full with water then air may leak in through the condensate path. That'll cause the inducer to draw a little less vacuum in the collector box and in some atmospheric conditions (and elevations) the inducer pressure switch might not operate as expected, causing the furnace to shut down.

  • too little condensate in the trap, +1. There's a little stupid looking box that two drains go into, and one comes out. That's either empty and needs to be primed.... or it doesn't exist. Or the AC condensate also drains into it and doesn't have a tee instead of an elbow for an air inlet (outlet) at the top of the line where it changes direction to go down, which will blow the drain assembly dry. - Take one of the rubber plugs out towards the top of the drain assembly and fill it with a turkey baster. It's part of the start up instructions.
    – Mazura
    Jun 9, 2023 at 3:49
  • On an old Goodman, it's the grey plastic box on the outside with drains coming and going; just pour some water in. Modern ones (and basically every other manufacturer on the planet) it's inside, and you take the top plug out to fill it, then put it back.
    – Mazura
    Jun 9, 2023 at 3:55
  • I do have that "stupid looking box" and will try what you and Greg Hill have suggested before I attempt to pull any clogs out with wet vac. If it is a priming issue, I don't understand why the burners will eventually light, often after several attempts. Once this P-trap is primed, what might cause it to lose prime later on?
    – John
    Jun 11, 2023 at 19:05
  • I've ruled out a priming issue by pushing water into the P-trap box through the tubes from the heat exchanger and the inducer until water flowed freely out of the main condensate drain. Unfortunately, ignition remains intermittent. Next step is to attach a wet-vac to the main condensate drain in an attempt to remove any clogs that might be preventing the system from creating a sufficient draft. Judging from the sound of the inducer motor, I suspect there may be some water trapped in the inducer housing. If so, will the wet-vac pull that out or will it need to be disassembled?
    – John
    Jun 19, 2023 at 12:02
  • @John A small amount of water in the housing/collector box is normal. If you hear it splashing as the inducer fan hits water that's definitely a problem. But a failure in the draft inducer system should be detected and halt the startup sequence before HSI even powers on -- it seems a stretch to imagine inducer trouble is causing intermittent ignition. I'm still thinking fuel supply/valve problem. Or maybe there are multiple troubles.
    – Greg Hill
    Jun 19, 2023 at 20:35

It really sounds like your flame sensor is dirty or bad. Most likely it just needs to be cleaned. It looks like a small probe that goes into the combustion chamber, it will have a small wire connected to it. It is usually held in place with one screw. Remove it and clean with steel wool. Don't touch the probe with your fingers, the oil from your skin will make it get dirty again faster. If that doesn't work it may need to be replaced, but a cleaning almost always solves the issue.

  • Thank you for your input! However, I believe I've ruled out any issue with the flame sensor on my system.
    – John
    Jun 7, 2023 at 16:18

Intermittent function possible causes

Could be Flame sensor- verified by OP it is not since it works

Could be the igniter- verified by OP it is not since it works

Last not least the condensate drain is clogged (slow drain) and the overflow switch shuts down the furnace.

There are number of videos on youtube how to clean it

  • I've watched these videos and will attempt to vacuum out the P-trap and hoses as shown. Thank you! I also noted that the AC condensate drains directly into the pump without the benefit of its own P-trap. The outlet of that PVC drainpipe may at times be either above or below the water level in the pump. I'm not sure why that would affect the furnace condensate drainage issue, but I plan to install a P-trap in the AC drain anyway.
    – John
    Jun 7, 2023 at 16:16

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