Goodman gas furnace limit switch opening, blower only runs with a/c

Question

I apologize in advance for some of my incorrect terminologies. 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.

• 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 a 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 the thermostat is set to cool or off with the 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 a 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 the furnace was serviced.

It's 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 was also turned on. The furnace ignited, blower continued to run for about 30 seconds at which time the blower shut off, 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 thorough will help someone with the proper knowledge point me in the right direction & we can finally put this problem behind us.

Answer 1

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,