||Either the circuit breaker has shorted or the unit's fuse has blown.
||Reset the circuit breaker associated with your Coleman furnace. If it's still not heating correctly, you'll want to replace the fuse.
||The igniter has cracked or become defective.
||Locate the igniter within your Coleman furnace and inspect the part for cracks. If it's damaged then you'll have to replace the part. Otherwise, test it with a multimeter for continuity to see if it's defective.
||Your thermostat might be calibrated improperly and as a result, is adjusting incorrectly.
||Check to see if your thermostat has an adjustable calibration scale, if so then use this to adjust the heat anticipator. If this doesn't appear to work or you don't have a scale, replace the thermostat completely.
|Not blowing hot air?
||The blower motor in your Coleman furnace is either burnt out or it's connecting belt has broken.
||You'll have to inspect the motor to see if power is reaching it. If it does but the motor continues to function incorrectly, then the blower motor has failed. However, if the motor continues to run, but the air still isn't moving, then the connecting belt has broken. Depending on the situation, you'll have to replace the motor or the belt.
|Blowing too much air?
||The control board is experiencing difficulty. The board's relay that regulates how much voltage is sent to the blower motor is defective.
||You will have to replace the control board.
||The contact on your thermostat responsible for controlling the blower motor is defective.
||Use a multimeter to check for continuity, if the thermostat isn't working properly then replace it.
|Starting and then stopping shortly after?
||The flame sensor used to detect whether or not a flame is present requires attention. The sensor has become either dirty or defective.
||Use a fine abrasive pad to clean the sensor if it appears to be dirty. If it's defective then replace the sensor altogether.