Central AC keeps running after the thermostat is satisfied or turned off
Separate two complaints first: a system that runs very long cycles on a hot day but still cycles off is usually a capacity, airflow or load problem; a condenser that never stops, even with the thermostat off, is usually an electrical fault. The classic electrical cause is a contactor whose contacts have welded closed, followed by a shorted thermostat cable that keeps 24 V on the Y circuit. Isolate which one it is before replacing anything.
Before you start
- The outdoor unit has line voltage (typically 240 V) even when the contactor is open; pull the outdoor disconnect and verify dead before touching the contactor or capacitor
- Discharge the run capacitor before working near it
- No refrigerant work: if the cause turns out to be low charge or a sealed-system issue, refer to an EPA 608 certified technician
Diagnostic steps
Unlock all 5 steps with expected values
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