||Either the furnace's electrical system has shorted, or the power switch has blown.
||To fix this, you'll have to reset your furnace's circuit breaker. However, if the switch has blown, you'll have to replace it.
||The igniter in your furnace is either cracked or defective.
||Check the igniter for cracks, if you find some then you'll have to replace it. Otherwise, test the igniter with a multimeter for continuity to see if it's defective.
||The furnace's thermostat is adjusting incorrectly.
||If your thermostat has an adjustable calibration scale, use it to adjust the heat anticipator. If this doesn't work or you don't have a scale, you should replace the thermostat altogether.
|Not blowing hot air?
||The furnace's blower motor has either burnt out or is running improperly.
||If you notice that power's reaching the motor but it still isn't operating, you'll have to replace it. However, if it runs but the air doesn't move, the connecting belt has broken. You should replace the belt.
|Blowing too much air?
||The furnace is sending an improper amount of voltage to the blower motor due to a failed relay on the control board.
||You should replace the control board.
||The contact that controls the blower motor on your thermostat is stuck.
||You should use a multimeter to check for continuity. If it isn't operating correctly, you'll want to replace the thermostat.
|Starting and then stopping shortly after?
||Your furnace's flame sensor is either dirty or defective. This means that it's unable to sense whether a flame is present.
||You should use an excellent abrasive pad to clean the sensor if it's dirty. If it isn't dirty, you must replace the flame sensor entirely.