Symptoms
- Indoor fan runs but no fan or hum from the outdoor unit
- No cooling produced at all
- Outdoor unit may be completely silent or humming without running
Likely causes
- CommonTripped breaker or blown disconnect fuse for the outdoor unitThe outdoor unit typically has its own breaker or a pull-out disconnect box nearby.
- CommonThermostat not calling for cooling properlyA wiring or setting issue at the thermostat can prevent the outdoor unit from ever getting a signal.
- CommonFailed capacitorA weak or dead capacitor prevents the compressor and fan motor from starting, sometimes producing a hum with no movement.
- PossibleFailed contactorThis relay switches power to the outdoor unit and can fail, leaving it with no power at all.
- Less commonCompressor failureA seized or burned-out compressor won't run regardless of other components.
What to do
- Check the breaker panel for a tripped breaker feeding the outdoor unit.
- Find the outdoor disconnect box (usually a gray box on the wall near the unit) and confirm the pull-out fuse block is fully seated.
- Confirm the thermostat is set to Cool and calling for a temperature below the current room reading.
- Listen closely at the outdoor unit; a humming sound with no movement suggests a bad capacitor or seized motor rather than no power at all.
- Don't repeatedly reset the breaker if it trips again immediately.
- Call an HVAC technician to test the capacitor, contactor, and compressor if power checks out but the unit still won't run.
Tools and parts
- Flashlight
AC run capacitorSearch: “air conditioner condenser run capacitor replacement”
When to call a pro
Call an HVAC technician to test the capacitor, contactor, and compressor if the outdoor unit stays silent after checking the breaker and disconnect.
Safety
Use caution
Never open the outdoor unit's electrical panel or touch a capacitor yourself; it can hold a dangerous charge even when the power is off.