Outside unit not running
If the indoor fan blows but the outdoor unit sits silent, the house won't cool, and with a heat pump it won't heat well either.
Safe checks
- Check the breaker for the outdoor unit in your panel. It's usually a double breaker labeled AC, HP or condenser.
- Check the disconnect box on the wall next to the outdoor unit. Make sure it's in the on position.
- Confirm the thermostat is calling for cooling or heating, not just running the fan.
- Look at the indoor unit for a full drain pan. A float switch may have shut the system down.
Hum but no spin?
If the outdoor unit hums but the fan doesn't turn, turn it off. A failed capacitor is the most common cause, and running it this way can damage the compressor. It's a fairly quick repair for a technician.
Other common causes
A burned contactor, a tripped high-pressure switch, a failed fan motor, or a wire chewed by a critter. Breakers that trip again right after resetting mean something is shorting, so don't keep resetting them.