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I apologize in advance for some of my incorrect terminology. My father has a 2004 Goodman GMH80904BN gas furnace with a/c. The a/c has functioned properly all summer here in Ohio, we have recently had some cooler nights and when attempting to switch from cool to heat and setting the thermostat well above the current temperature. The thermostat calls for heat, the exhaust/flue fan starts and shortly after the igniter glows and ignites the gas.The fire continues to burn for roughly a minute and then the gas shuts off while the exhaust/flue fan continues to run. Throwing a trouble blink code (4 flashes) which indicates High limit open.

This all makes perfect sense that the high limit would open because at no time does the furnace blower attempt to run. Now the blower does function fine when thermostat is set to cool or off with fan set to on.

To be safe, I replaced the filter. We disconnected the vent pipe/flue to ensure there was no debris.

Checked for continuity at the high limit switch while it was hot/open (no) and again after it had cooled/closed (yes).

Purchased new thermostat to rule out the existing one as faulty. The new stat produced the same symptoms.

I was told the pcb was replaced at the very most 5 years ago along with a new blower motor when furnace was serviced.

Its my understanding that the blower in this furnace is controlled by the pcb and i'm beginning to fear the pcb has failed again. I attempted switching the fan from auto to on while also switching to heat. The blower was on, the vent motor also turned on. Furnace ignited, blower continued to run for about 30 seconds at which the blower the shut off and then after a short period of time the limit switch opened again and everything shut down.

Sorry for the long post, but hopefully my attempt at being through will help someone with the proper knowledge point me in the right direction & we can finally put this problem behind us. Any help is greatly appreciated! God Bless

  • How are the dip switches set on the control board? If it's set for two stage operation, it's possible that the med. Low speed on the blower is broken. If the burner runs too long without a blower on, it will overheat. – Tester101 Oct 30 '16 at 17:18
  • The first switch says 2nd stage delay which is set to "auto" the second switch is set to 1stg not 2stg & the 3rd switch heat off delay is set to 150 sec – Loudrnu Oct 30 '16 at 22:43
  • After my father sent me a pic of the switch settings I suggested he try flipping the middle switch from 1stg to 2stg. He says everything seems to be working now as it should, blower is now running with heat which it wouldn't start on 1stg. So does this mean only the lower speed on the blower is working and when the furnace attempts to ramp up to the 2nd stage the blower will possibly shut off & limit will eventually open? – Loudrnu Oct 30 '16 at 22:59
  • that would be my guess. You can test it, by temporarily swapping the motor leads, or powering the various speeds manually. – Tester101 Oct 30 '16 at 23:03
  • He just updated me and said that after a bit of running ok while set to 2stg, it seemed like it attempted to ramp up to the high part of 2stg and the blower shut off while fire continued and limit switch opened, fire shut off and 4 flashes on the led for open limit again. – Loudrnu Oct 30 '16 at 23:15
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Some of the older Goodman furnaces was equipped with a flame sensor it was just a solid metal probe with one attached wire and when it heated up it caused a small millivolts in the probe if this sensor fails your furnace will not light or will go out after a few seconds, only maintenance required is cleaning the metal probe with steel wool or sand paper and check for a loose terminal due to heat stress its cheap and inexpensive to replace so it should be replaced every 5 years most problems with these probes were burned wires and connections as they are directly in contact with the burner (NOTE) Goodman furnaces were designed to run a few seconds in order to give the flame sensor time to heat up and produce the electrical current to activate the switch if no activity is sensed the burner will shut off, a flame sensor works on the same principal as an expansion thermo coupling except there is no movement on a flame sensor it works on millivolts of current generated by heating metal where as a thermo coupling has a mechanical rod in the housing that moves with heat expansion and stays as long as the pilot is burning,

  • Doubt this is a flame sensor problem since the burner is clearly lighting off without a hitch... – ThreePhaseEel Nov 4 '17 at 3:32
  • it will light then go out in a few seconds if flame sensor fails to deliver the signal to the gas valve it will close the temporary circuit and shut down same as any thermo coupling on any other device – user77343 Jan 11 '18 at 0:35

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