Learn why an outdoor air-conditioning unit may not run even when the thermostat is calling, including power, controls, contactor, capacitor, fan and compressor issues.
The outdoor unit contains the compressor and condenser fan, so cooling capacity is lost if it does not operate during a normal cooling call.
An indoor blower running by itself can make it seem like the AC system is operating even though the refrigeration circuit is not.
Outdoor-unit troubleshooting can involve line voltage and stored electrical energy, so homeowner checks should remain limited.
A breaker, disconnect, fuse or wiring issue can interrupt power.
Thermostat, control-board, float-switch or low-voltage problems can prevent the contactor from being energized.
A weak or failed capacitor can prevent a fan motor or compressor from starting.
A failed or damaged contactor can prevent line voltage from reaching components.
The compressor may overheat or shut down if the outdoor fan is not moving air.
Overload, internal protection or compressor faults can stop operation.
Several different HVAC faults can produce the same homeowner symptom. Proper diagnosis may require electrical readings, refrigerant measurements, airflow testing, static-pressure measurements, combustion testing or control-sequence checks.
Do not bypass safeties, open sealed refrigerant circuits, work on gas piping or perform live electrical troubleshooting unless you are qualified to do so.
An inactive condenser is a common cause.
A missing control call can keep the outdoor unit off.
Humming or buzzing can accompany start-component problems.
Understand the outdoor unit's role in the refrigeration cycle.
When requesting service, include the equipment type, approximate age, when the problem started, whether the system still operates and any unusual noise, water, odor or thermostat behavior you have noticed.
Understand contactors, capacitors, voltage and control circuits.
Follow the control path that starts a cooling call.
Look up common outdoor-unit electrical and refrigeration parts.
Understand normal system operation before narrowing a symptom.