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.

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.

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.

Control Board
The control board regulates the power supply to all of the components of the furnace. If the control board fails, it might not send voltage to the ignition system, causing the furnace not to heat.

Flame Rollout Limit Switch
The flame rollout switch monitors the heat surrounding the burners. If the inducer fan motor cannot draw enough air through the burners, or if the furnace is not venting properly, the rollout switch will open to halt the ignition sequence and prevent the furnace from heating. To determine if the flame rollout switch is defective, use a multimeter to test it for continuity.If the switch does not have continuity and the reset button is not tripped replace it. Be aware that poor airflow through the burner is often a result of insufficient ventilation or a restricted exhaust vent. Make sure that your furnace is in a properly ventilated location and the exhaust vent is clear of obstructions.

Wall Thermostat
The wall thermostat has electrical contacts that control the power supply to the furnace. If the contacts in the thermostat fail, the furnace will not turn on. To determine if the thermostat is defective, use a multimeter to test it for continuity. If the wall thermostat does not have continuity, replace it.

Gas Valve Assembly
The gas valve opens to allow gas to flow into the burner. If the gas valve is defective, the valve will not open, preventing the furnace from heating. To determine if the gas valve is at fault, use a multimeter to test it for continuity. If the gas valve does not have continuity, replace it.

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. 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.

SOURCE: repairclinic.com

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.

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.

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.

Control Board
The control board regulates the power supply to all of the components of the furnace. If the control board fails, it might not send voltage to the ignition system, causing the furnace not to heat.

Flame Rollout Limit Switch
The flame rollout switch monitors the heat surrounding the burners. If the inducer fan motor cannot draw enough air through the burners, or if the furnace is not venting properly, the rollout switch will open to halt the ignition sequence and prevent the furnace from heating. To determine if the flame rollout switch is defective, use a multimeter to test it for continuity.If the switch does not have continuity and the reset button is not tripped replace it. Be aware that poor airflow through the burner is often a result of insufficient ventilation or a restricted exhaust vent. Make sure that your furnace is in a properly ventilated location and the exhaust vent is clear of obstructions.

Wall Thermostat
The wall thermostat has electrical contacts that control the power supply to the furnace. If the contacts in the thermostat fail, the furnace will not turn on. To determine if the thermostat is defective, use a multimeter to test it for continuity. If the wall thermostat does not have continuity, replace it.

Gas Valve Assembly
The gas valve opens to allow gas to flow into the burner. If the gas valve is defective, the valve will not open, preventing the furnace from heating. To determine if the gas valve is at fault, use a multimeter to test it for continuity. If the gas valve does not have continuity, replace it.

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. 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.

SOURCE: repairclinic.com