Originally Posted by kess
The "auto-on" switch on your thermostat controls only the fan. In auto mode, the fan cycles with the heater or air conditioner, in "On" state, well... it's *on*-regardless of call for heat or cooling.
First-always first-check your filter. If it was packed, this may have contributed to or caused the problem.
You say the furnace fan comes on-do you mean the blower or the little motor at the top of the burner (inducer)? If the blower comes on and the fan switch on the stat is in "Auto" then you likely have a limit tripped-something overheated or failed, and the furnace is out on limit.
Your problem is most likely in the hot surface ignitor. Set the temperature to 75 degrees (or 10 degrees higher than room temp). Turn heat on at stat. Go to furnace. Shut off furnace at service switch. Turn back on, wait for heat to cycle. Small inducer should start to run, followed by hot surface ignitor glowing orange, followed by main gas valve, and 1-2 minutes later fan.
See where it's not happening. Shut down furnace. Unplug ignitor (ceramic base has 2 wires coming out terminating in a plug) from matching plug. Turn furnace on, check for 120 volts across pins feeding ignitor. If 120 volts is present, but no glow-its the ignitor. (A quick look tells many times-a "hot spot" or bright white mark on the ignitor indicates a blowout.) If you don't get 120 volts to the ignitor, check condensate traps and clear them, pull tubing on pressure switch and clear. If water in pressure switch tubes-it's a condensate problem.
Also (but less common) note that Goodman had a problem back in the 90's with "crust" developing in the tubing connection of the inducer housing. The port for the pressure tubing would get clogged with combustion by products and eventually clog-killing the pressure switch action. Simply running a 1/8" drill bit through the port usually fixed the problem-for good. This may or not be applicable, as I haven't had to work on one in several years, and it's probably been remedied by now. Just a thought.
Afterthought:There should be a small red LED flashing in the lower compartment. Cycle the furnace, and count the flashes, the compare to the diagnostic code-usually printed on inside of cover. This will tell you in which area the failure occurred.
Hope this helps.