||Your furnace either has a blown fuse or a shorted circuit breaker.
||Reset your furnace’s affiliated circuit breaker or replace the blown switch entirely.
||The furnace’s igniter has either cracked or is now defective.
||Check to see if the igniter is cracked. Replace it if it is. If the piece isn’t cracked, test it with a multimeter to see if it’s defective.
||The thermostat is likely not adjusting correctly.
||Use the adjustable calibration scale on your thermostat to adjust the heat anticipator. If this doesn’t work or isn’t an option then replace the thermostat.
|Not blowing hot air?
||There is likely an issue with your furnace’s blower motor. It could be burnt out, the wheel could be defective, or the connecting belt may have snapped.
||If the motor shows to be receiving power but is still not running then replace the blower motor. But, if it does run, you will likely have to replace either the connecting belt or the wheel.
|Blowing too much air?
||Your control board isn’t functioning properly. The relay that regulates the blower motor’s voltage has failed.
||You will have to replace the control board.
||The contact that controls the blower motor function on your thermostat is stuck in a closed position.
||Use a multimeter to check for continuity, if this appears to be the issue then replace the thermostat.
|Starting and then stopping shortly after?
||Your furnace’s flame sensor isn’t able to sense the presence of a flame due to it being dirty or defective.
||Use a fine abrasive pad to clean to the sensor clean, if this doesn’t fix the issue then replace the flame sensor entirely.