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Air Conditioner Compressor Won't Run

The part(s) or condition(s) listed below for the symptom Air conditioner compressor won't run 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.

Most Frequent Causes for Air conditioner compressor won't run

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Cause 1Temperature Control Thermostat

The temperature control thermostat has electrical contacts inside that burn out over time. It is possible for the thermostat to still turn on the fan motor but not the compressor motor.

Parts
Cause 2Temperature Control Board

The temperature control board provides voltage to the fan motor and compressor. If the control board is defective, it may stop providing voltage to the compressor. Control boards are often misdiagnosed. Before replacing the temperature control board, make sure you check more commonly defective parts—particularly the compressor overload, capacitor and the compressor itself. If you are certain that all of the other components are working properly, replace the temperature control board.

Parts
Cause 3Thermostat

The thermostat monitors the temperature of the air. When the air temperature rises above a set point, the thermostat activates a switch to provide power to the fan and compressor. If the thermostat is defective, it may prevent the compressor from running. If you determine that the compressor is at fault, replace it.

Parts
Cause 4Overload

The overload protects the compressor from burning out if it draws too much current. Over time, the overload can burn out. To determine if the overload is burned out, use a multimeter to test the overload for continuity. If the overload does not have continuity, replace it.

Parts
Cause 5Compressor

The compressor itself might be defective. However, this is not usually the case. Before replacing the compressor, be sure to check more commonly defective components—particularly the overload protector and the compressor capacitor. If the compressor is defective, it should only be replaced by a licensed technician.

Parts
Cause 6Thermistor

The thermistor is a sensor connected to the control board that detects the temperature of the outside air. If the thermistor does not have continuity, it can prevent the compressor from running. To determine if the thermistor is at fault, use a multimeter to test it for continuity. If the thermistor does not have continuity, replace it.

Parts
Cause 7Relay Board

The relay board provides voltage to many of the components of the air conditioner. If the board is defective, it can stop sending voltage to the compressor. Relay boards are often misdiagnosed; before replacing the board, make sure you check more commonly defective parts—particularly the overload protector, the compressor capacitor, and the compressor itself. If you are certain that all of the other components are working properly, replace the relay board.

Parts
Cause 8Main Control Board

The main control board provides voltage to all of the components of the air conditioner. If there is a problem with the main control board, it might not send voltage to the compressor. However, this is rarely the case. Before replacing the board, make sure you check more commonly defective components—particularly the overload protector, compressor capacitor, and the compressor itself. If you are certain that all of the other components are working properly, replace the main control board.

Parts
Cause 9Capacitor

The capacitor is attached to the compressor by electrical leads. If the capacitor burns out, the compressor won’t run. To determine whether the capacitor is burned out, use a multimeter to test it for continuity. If the capacitor does not have continuity, replace it.

Parts
Cause 10Selector Switch

The selector switch directs voltage to the compressor. If the selector switch is defective, the compressor won’t run. First, check the capacitor, overload, and the compressor itself. If none of these parts are defective, the selector switch might be at fault. If the compressor runs intermittently when you press the selector buttons or rotate the switch knob, replace the selector switch.

Parts