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HomeRepair HelpCentral A/C RepairCentral air conditioner won't turn on Repair
Central air conditioner won't turn on
If your central air conditioner won't turn on, the problem is usually in the low-voltage control circuit, power supply, or thermostat rather than the compressor or condenser fan. The most likely cause is a failed furnace or air handler transformer, which should send 24 volts to the thermostat and contactor coil when cooling is requested. Next, a defective furnace control board may prevent the contactor from receiving its 24-volt signal or stop power from reaching the blower motor. If the thermostat display is blank, replace its batteries when applicable. Then check for incoming power, a tripped circuit breaker, blown fuse, or the unit power switch being turned off. A faulty wall thermostat or thermostat wiring is less common but can also prevent the AC from starting. Testing thermostat wire terminals with a multimeter can help confirm the problem, but work inside the disconnect box or on 240-volt wiring is not a safe DIY repair. Turn off power before inspecting components, and have a qualified HVAC technician handle line-voltage diagnosis or replacement. Enter your model number to find the exact transformer, control board, thermostat, or other replacement part for your central air conditioner.
01 - Central Air Conditioner Transformer
The furnace control board sends a low voltage signal of 24 volts to the thermostat. When the themostat cooling contacts close, a 24 volt signal is sent to the contactor coil, closing the high voltage circuit and sending power to the condensing unit components. Some furnaces have a separate transformer for the 24 volt circuit, so if the contactor is not receiving the proper voltage when the thermostat cooling contacts are closed, it's likely this transformer has failed and the component will need to be replaced.
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Highly Likely
02 - Central Air Conditioner Control Board
If the central air condensing unit contactor is not receiving 24 volts or power is not getting to the furnace blower motor, a possible cause is a defective furnace control board. While this is not a common problem, if all other potentially faulty components are working properly, the control board should be replaced.
Highly Likely
03 - Wall Thermostat Batteries
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.
Common
04 - Incoming Power Problem
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.
Common
05 - Central Air Conditioner Wall Thermostat
The furnace control board sends a low voltage signal of 24 volts to the contactor through the wall thermostat. When the contactor receives the signal from the wall thermostat, it closes the circuit, sending voltage to the condensing unit components. There should be 24 volts between the common (“C”) and yellow (“Y”) thermostat wire terminals on the furnace control board when the wall thermostat is in the cooling mode. You can check for this voltage with a multimeter. If voltage is not reaching the contactor, it's likely the wall thermostat is defective and will need to be replaced.
Less Common
06 - Central Air Conditioner Wifi Thermostat
The furnace control board sends a low voltage signal of 24 volts to the contactor through the WiFi thermostat. When the contactor receives the signal from the WiFi thermostat, it closes the circuit, sending voltage to the condensing unit components. There should be 24 volts between the common (“C”) and yellow (“Y”) thermostat wire terminals on the furnace control board when the WiFi thermostat is in the cooling mode. You can check for this voltage with a multimeter. If voltage is not reaching the contactor, it's likely the WiFi thermostat is defective and will need to be replaced.
Less Common
Frequently Asked Questions
Why is my central air conditioner not turning on?
Why is my AC thermostat not turning on the air conditioner?
How do I know if the central air conditioner transformer is bad?
Can a bad furnace control board keep the outdoor AC unit from turning on?
How do I find the correct replacement part for a central air conditioner that won't start?
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