I have a comfortmaker furnace about 15 years old. When I turn the heat on it works just fine the gas comes on and the blower blows warm air into the house. When I turn the air conditioner on the compressor and outside fan come on but the blower in the furnace will not come on. I tried the fan on with the thermostat switch and still no air. I tried another thermostat and still could not could the blower to turn on. I am woundering if it is the control module in the furnace. It's a General 90 model 50A50-110.

Could be the fan relay if so equipped.