Cause 1

Control Board

The control board regulates the power supply to all of the components of the furnace. If the control board is defective, it might not send voltage to the blower motor.

Cause 2

Capacitor

If the capacitor is defective, the fan motor might be noisy, overheat, or not run at all. To determine if the capacitor has failed, inspect the capacitor. If the capacitor is bulging or leaking, replace it. The capacitor can also be checked with a meter.

Cause 3

Blower Motor

The blower motor blows air through the return vent, past the heat exchanger, and into the home’s venting. First, make sure that power is getting to the motor. If power is getting to the blower motor, but the motor does not run, the motor may be burned out. If the blower motor is burned out, replace it. If the blower motor hums, check the blower fan to ensure that it spins freely. If the blower fan is unobstructed but does not spin freely, replace the blower motor.

Cause 1

Draft Inducer Motor

The draft inducer motor draws air into the burner and then exhausts it out the flue. If the motor bearings are worn out, the motor will be loud. If the draft inducer motor is unusually noisy, replace it.

Cause 2

Blower Motor

Over time, the bearings in the blower motor wear out, causing the motor to become noisy. Some furnace motors can be lubricated—if your furnace motor can be lubricated, try lubricating it first. If lubricating the motor doesn’t solve the problem, or if your motor can’t be lubricated, replace the motor.

Cause 3

Blower Wheel

The blower wheel blows air out of the furnace and through the house. The blower wheel is attached to the blower motor shaft with a set screw. If the set screw is loose, the blower wheel will wobble on the motor shaft, and the wheel will be noisy as it turns. Check the set screw to ensure that it is properly tightened. Next, inspect the wheel for damage. If the blower wheel is damaged, replace it.

Cause 1

Wall Thermostat

The wall thermostat has electrical contacts that control the power supply to the furnace. If the contact that controls the blower motor sticks closed, it will send continuous voltage to the blower motor, causing the motor to run continuously. To determine if the thermostat is defective, use a multi-meter to test it for continuity. If the wall thermostat shows continuity between the (R) and (G) terminals when the thermostat is in the off position the thermostat has failed and will need to be replaced.

Cause 2

Control Board

The control board has relays that regulate the voltage to each component in the furnace. If the relay that controls the blower motor fails while it is closed, it will send continuous voltage to the blower motor, causing the motor to run continuously. If the control board is at fault, replace it.

Cause 1

Draft Inducer Motor

The draft inducer motor draws air into the heat exchanger and then exhausts it out the flue. The pressure switch senses a pressure change and closes a switch to signal the control board that the furnace has proper air flow. If the draft inducer motor is defective, it may be unable to close the pressure switch, causing the ignition process to stall and the furnace to shut off after a few minutes. If the ignition process stalls, the furnace will shut off. If the draft inducer motor is at fault, replace it.

Cause 2

Flame Sensor

The flame sensor monitors the burner to detect whether or not a flame is present. If the flame sensor is defective, it might fail to detect a flame. If the flame sensor does not detect a flame, the control board will shut off voltage to the gas valve to prevent the furnace from heating. Sometimes, if the flame sensor is dirty, it will fail to detect a flame. Try cleaning the flame sensor with a fine abrasive pad. If the furnace still doesn’t heat, replace the flame sensor.

Cause 3

Igniter

The igniter gets very hot and glows bright orange to light the gas burner. If the igniter fails or cracks, the furnace won’t heat. To determine if the igniter is faulty, remove the igniter and inspect it for cracks. If the igniter is cracked, replace it. If the igniter is not cracked, use a multimeter to test the igniter for continuity. If the igniter does not have continuity, replace it.

Cause 1

Pressure Switch

The pressure switch closes when the burner has sufficient airflow. If the pressure switch isn’t working, it might not close when the proper airflow is present, causing the ignition process to stall. If the ignition process stalls, the furnace will shut off. To determine if the pressure switch is defective, use a multimeter to test the switch for continuity. If the pressure switch does not have continuity, replace it.

Cause 2

Control Board

The control board regulates the power supply to all of the components of the furnace. If the control board is defective, it might not send voltage to the ignition system, causing the furnace to shut off after a few minutes.

Cause 3

Flame Sensor

The flame sensor monitors the burner to detect whether or not a flame is present. If the flame sensor is defective, it might fail to detect a flame. If the flame sensor does not detect a flame, the control board will shut off voltage to the gas valve to prevent the furnace from heating. Sometimes, if the flame sensor is dirty, it will fail to detect a flame. Try cleaning the flame sensor with a fine abrasive pad. If the furnace still doesn’t heat, replace the flame sensor.