Furnace not turning on

01 - Check Fault Codes

If the furnace or air handler stops working properly, the control board will often display a fault code to help diagnose the problem. The fault codes are displayed by a steady or flashing light on the control board. The lights indicate either an error code or the status of the furnace or air handler. Some manufacturers will use red and green lights to indicate specific fault codes. Fault code descriptions are often displayed on the access cover of the unit for easy reference.

Highly Likely

02 - Furnace Transformer

A furnace transformer is the component that decreases or increases the incoming voltage to power the unit. A faulty transformer can prevent the furnace from turning on. If there is no secondary 24 volt power to the thermostat, the furnace trips a circuit breaker when it comes on, or you hear a consistent humming or vibrating noise, it's possible the transformer has failed.

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Highly Likely

03 - Wall Thermostat Batteries

Most wall thermostats are either powered by an enclosed battery or by the furnace transformer. If the thermostat display is dead, you can remove the thermostat cover and look for replaceable batteries. Replace the batteries if needed and check operation.

Highly Likely

04 - Furnace 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 to not turn on.

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Common

05 - Furnace Main Control Board

The main 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 to not turn on.

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Common

06 - Furnace Wifi Thermostat

The WiFi thermostat is responsible for controlling components and regulating the home's temperature. The thermostat is usually powered with 24 volts from a control board on a separate unit like a furnace or air handler. The thermostat also has terminals to receive and distribute low voltage 24 volt power to run different HVAC products such as a furnace, air conditioner, or heat pump. If the WiFi thermostat is receiving power but the display is dead or it will not send power out in the appropriate modes, the WiFi thermostat will need to be replaced.

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Common

07 - Furnace Wall Thermostat

The wall thermostat has electrical contacts that control the heating, cooling and fan modes of 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 in the respective modes. If the wall thermostat does not have continuity, replace it.

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Common