The blower motor on my Janitrol GMP 125-4 furnace cycles without being called for by the thermostat. There is no pattern to it turning on and off. It turns on and cycles even with the thermostat set to off. The thermostat is not the problem. I disconnected the green wire from the thermostat and it still will cycle. Being summer the air is now being used and it is happening with the thermostat set to cool. There is no blinking light on the circuit board that would indicate an open limit. This is not a dual speed fan so when it comes on the fan is at full speed. It may happen once aday, once an hour or once ever few days, again no pattern. The air system is cycling normally. Thermostat calls for air, the AC unit turns on and the fans cycles normally. Any ideas?
I think you might be having an intermitent power problem. Some gas furnaces have a purge cycle they run through on power-up which includes the blower motor running for about 90 seconds and then shutting down. The Goodman GMP service access doors are made with very thin metal which could be vibrating a defective door switch on the furnace. Try removing the lower service door and direct connect the two leads on the door switch with the circuit breaker off. Power up the furnace and see if it happens again over a weekend. If it does re-install original door switch and replace fan control board. If it does not, Install a replacement door switch.