The part(s) or condition(s) listed below for the symptom Furnace not turning on are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
Furnace Blower Not Running? Gas Furnace Troubleshooting
Furnace Blower Runs All The Time? Gas Furnace Troubleshooting
Furnace Noisy Or Loud? Gas Furnace Troubleshooting
Furnace Starts Then Stops? Gas Furnace Troubleshooting
Furnace Not Heating? Gas Furnace Troubleshooting
Replace Furnace Filters - Tech Tips from Repair ClinicIf 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.
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.
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.
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.
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.
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.
Check to see if there is power to the furnace. One or more indicator lights on the control board should be illuminated. If there is no power to the furnace, check the home's furnace circuit breaker or fuse. Confirm the power switch on the side of the furnace is turned on.
