Hello, I'm hoping someone can help me figure out what's wrong with my furnace. I believe it is a Goodman GMP/GMPE gas furnace but I'm not sure snce I can't find that information anywhere on the appliance. About a week ago it started blowing out only cold air. I though it might be a thermostat issue so I replaced the old thermostat with a new electronic one but that has not solved the problem. The pilot light goes on and it goes through it's cycle of (supposedly) heating up. The last few days it's sometimes blown hot air, sometimes warm air and sometimes cold air. What could be the problem? Thank you.

Metal, or PVC flue/exhaust? Look for a status LED behind a clear viewer in the front panel.

There is a red light near the bottom behind the clear viewer. I believe it's a metal exhaust. There is a large metal tube coming out of the top.

If the LED is steady, set thermostat to demand heat & observe LED. Blinking indicates a problem. Observe/note pattern.

I tried that and the light stays steady.

Try this: turn off power & disconnect wire from "R" terminal at thermostat. Turn power back on. What happens?

There is no 'R' terminal. Only 'RC' and 'RH' and they're attatched by a jumper wire.

The jumper is good, but there should be another wire connected to either "RC" or "RH". Usually red.

Okay, so disconnect the wire coming from the wall ('RC') from the thermostat and then turn it back on, right?

Yes. Leave thermostat off base for now, and make sure "R" wire (only 24 volts) isn't touching anything.

I will try that and let you know if it works in a little while. Thank you for all of your help, I hope this fixes it!

No, this isn't a FIX, this is a STEP. Toward, hopefully, a FIX.

Thanks for all of the help KC13 but, unfortunately, it looks like I'm going to have to buy a new furnace altogether.