We have Janitrol / Goodman GMP Series gas furnace. Recently, the furnace would turn off before set temperature on the thermostat. I had a Lux programmable thermostat and thought this was the problem. I replaced the thermostat with a Honeywell programmable thermostat and it worked great. The heat turned on and off as desired for one day. Then this morning we are back to an indoor temperature reading below the desired temperature (which for us is 70) on our new thermostat. So it is not the thermostat. Last year we replaced the motor, capacitor and the venter blower assembly - so it is not these parts. It seems to be an electrical problem. Any information will be appreciated.

Any codes flashing on the circuit board?

Make sure return air filter is clean. If it is dirty and restricting air flow it will trip the limit.

First, thank you both for the quick response. A quick cleaning may have done the trick. I have a Janitrol / Goodman GMP 125-4 gas furnace with the original circuit board so I did not see any codes displayed but please advise if I missed it. The air filter was changed within the last month. I checked it again and there was no excessive dust collection. With that in mind, I removed the control access door and the blower door below it and noticed it was quite dusty. I carefully vaccuumed the entire area and so far it seems to be working fine. I am going to call for a professional cleaning / tune up tomorrow. In any case - I'll see if I wake up with an angry wife or a warm one.