Son in law has a Goodman natural gas furnace. No heat. LED is flashing 4 times ('Upper limit open' is the message associated with this error). The blower motor turns on, the exhaust motor (I think that's what it is) turns on for about 10 seconds then turns off.

Tried changing the air filter (was very dirty) but that didn't help.

Any other ideas? I don't have the full specs for the furnace right now.

Thanks,

Freezing in the north

Reset upper limit control. Wiring diagram on back side of blower door. Something caused this safety item to trip and that might have been the dirty air filter. Look for reset button in center on control. If it will not reset replace.

I went back to my son in laws to do more checking this morning and brought my meter. I found that the main limit control thermostat had indeed opened up (probably because of the dirty filter decreasing the air flow) and had not reset (and there was no reset button on it). As a temporary solution, I just gave the thermostat a sharp tap to reset it and it closed again so that the furnace could turn back on. Will get a new thermostat to replace that one. AND told them to replace the filter on a regular basis!!
Thanks for your reply hvac1000.

Replace with same value. Glad you got the heat going since it is getting cold out there.