Symptoms
- AC runs continuously regardless of temperature setting
- System doesn't stop even when set to Off
- House gets uncomfortably cold
Likely causes
- CommonThermostat sensor reading incorrectlyIf the thermostat thinks the room is warmer than it is, it will never be satisfied.
- PossibleStuck relay or contactor in the outdoor unitA physically stuck contactor keeps the compressor running regardless of the thermostat's signal.
- PossibleWiring issue bypassing the thermostat's controlCertain wiring faults can make the system run independent of the thermostat.
- PossibleThermostat set to a very low temperature by mistakeA simple, easily overlooked setting can cause the system to run far longer than expected.
What to do
- Double-check the thermostat's set temperature and mode, since a low set point will simply keep the AC running longer than expected.
- Try switching the system to Off directly on the thermostat and see if the AC responds.
- If it doesn't stop even in Off mode, turn off power to the outdoor unit at its breaker as an immediate way to stop it.
- Check the thermostat's temperature reading against a separate thermometer for accuracy.
- With power off, inspect (without touching live components) whether the outdoor unit's contactor appears stuck or burnt, and if so, leave power off until a professional can inspect it.
- Do not restore power to the outdoor unit until the cause is identified if the system ignored the Off command.
Tools and parts
- Separate room thermometer
When to call a pro
Call an HVAC technician promptly if the system won't turn off even from the breaker-adjacent switch or if a stuck contactor is suspected.
Safety
Use caution
If the AC won't stop even when set to Off, turn off the outdoor unit's breaker immediately rather than letting it run indefinitely.