||Either the furnace’s circuit breaker has shorted or the fuse is blown.
||You’ll have to replace the blown switch or reset the furnace’s circuit breaker.
||Your furnace’s igniter may be either cracked or defective.
||Should the igniter be cracked, you’ll have to replace it. Otherwise, test with a multimeter for continuity.
||The thermostat is likely not adjusting correctly.
||If your thermostat has an adjustable calibration scale, use this to adjust the heat anticipator. If this doesn’t work or isn’t an option, replace the thermostat.
|Not blowing hot air?
||Your unit’s blower motor has either burned out or the connecting belt has snapped.
||Troubleshoot to see if the blower motor is receiving power: if it isn’t running, the motor has failed, and you’ll have to replace it. If it does run, then the connecting belt has snapped, and you’ll have to replace the belt.
|Blowing too much air?
||The relay that regulates the amount of voltage that is sent to your furnace’s blower motor on the control board has failed.
||You will have to replace the control board.
||Your thermostat’s contact that controls the blower motor’s functions might be stuck in a closed position.
||You’ll have to 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 detects whether or not a flame is present. Should the sensor be dirty or defective, it will not be able to read a flame’s presence.
||If the flame sensor is dirty, use an abrasive pad to wipe it clean. If it’s defective, replace the flame sensor.