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HomeRepair HelpCentral A/C RepairCentral A/C Condensing unit does not run Repair
Central Air Conditioner Condensing unit does not run
Central air conditioner condensing unit not running means the outdoor unit is not starting, even if the indoor blower runs. The most likely cause is a failed compressor, which pumps refrigerant through the condenser coil and evaporator coil. A multimeter continuity test can help confirm the fault, but compressor replacement requires a qualified HVAC technician. Next, check the wall thermostat, since failed thermostat contacts can prevent both the furnace and outdoor condensing unit from running. A faulty condenser fan motor can also keep the AC condenser from turning on. With power disconnected, the condenser fan blade should rotate freely, while continuity and voltage tests can help identify a seized motor or electrical failure. If the unit has power but the contactor is sound and neither the compressor nor fan runs, low refrigerant may have tripped the low pressure switch, indicating a sealed-system problem. Pressure switches are less common causes, and replacement requires refrigerant recovery. Turn off power at the disconnect box and circuit breaker before testing. Do not work on 240V wiring unless qualified. Refrigerant or sealed-system repairs require EPA 608 certification and a licensed HVAC technician. Enter your model number to find the exact replacement part for your central air conditioner.
01 - Central Air Conditioner Compressor
Central air conditioners use a compressor to pump refrigerant through the condenser coils located in the outside condensing unit and through the evaporator coils mounted on top of the furnace or air handler. While it's not a common problem, if the compressor fails, the condensing unit will not run. You can use a multimeter to test the compressor for electrical continuity, a continuous electrical path present in the component, to help determine if the compressor is faulty. If all other potentially defective components, such as the capacitor and condenser fan motor, are operating properly, then a qualified technician should ...
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Highly Likely
02 - Central Air Conditioner Wifi Thermostat
The WiFi thermostat has contacts in it that control the furnace. If the contact fails it can result in the furnace and the condensing unit to not run at all.
Common
03 - Central Air Conditioner Condenser Fan Motor
A faulty condenser fan motor can be responsible for the central air conditioner condensing unit not running. You can use a multimeter to test the fan motor for electrical continuity, a continuous electrical path present in the motor, to help determine if the motor is defective. You can also use the multimeter to determine if power is reaching the motor. You can also try rotating the fan blades by hand. If the blades do not turn freely, it's likely the motor bearings have seized and the fan motor will need to be replaced.
Common
04 - Central Air Conditioner Fan Motor
A faulty fan motor can be responsible for the central air conditioner condensing unit not running. You can use a multimeter to test the fan motor for electrical continuity, a continuous electrical path present in the motor, to help determine if the motor is defective. You can also use the multimeter to determine if power is reaching the motor. You can also try rotating the fan blades by hand. If the blades do not turn freely, it's likely the motor bearings have seized and the fan motor will need to be replaced.
Common
05 - Sealed system problem
If the condensing unit has power to it, the contactor checks out okay, but the fan and compressor are not running, it is likely that the unit is low on refrigerant which will cause the low limit pressure switch to trip and keep the unit from running. Since diagnosing and repairing a sealed system takes specialized tools and knowledge, a qualified service technician should diagnosis and repair the unit.
Common
06 - Central Air Conditioner Pressure Switch
A pressure switch is a safety device used to help prevent component and system failures. There are basically three types of pressure switches used in central air conditioning and heat pump applications: High Limit, Low Limit, and Ambient Temperature. Depending on the model, you may have one or more of these or none. The pressure switch(es) can be checked with a multimeter, referencing the wiring diagram for the circuit locations. Note: If a pressure switch needs to be replaced the refrigerant will need to be removed, requiring a qualified technician.
Less Common
Frequently Asked Questions
Why is my central AC condensing unit not turning on?
How do I troubleshoot an AC condenser fan that is not running?
Can a thermostat or pressure switch keep the outside AC unit from starting?
What should I check before replacing the AC compressor?
How do I find the correct replacement part for an AC condensing unit that will not run?
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