The part(s) or condition(s) listed below for the symptom Central air conditioner won’t turn 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.
Central Air Conditioner Not Cooling? Central Air Conditioner Troubleshooting
Central Air Conditioner Noisy? Central Air Conditioner Troubleshooting
Central Air Conditioner Fan Motor Not Running? Central Air Conditioner Troubleshooting
Central Air Conditioner Not Running? Central Air Conditioner TroubleshootingTransformer
Control Board
Most wall thermostats are either powered by an enclosed battery or by the furnace or air handler 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.
Check to see if there is power to the furnace or air handler. One or more indicator lights on the control board should be illuminated. If there is no power to the furnace or air handler, check to see if a circuit breaker has tripped or a fuse has blown. Confirm the power switch on the side of the unit is turned on.
Wall Thermostat
WiFi Thermostat
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.
