When an AC compressor will not start, the fault is usually in the parts that start it, not the compressor itself: a failed run or start capacitor, a worn contactor, a tripped thermal overload, or low supply voltage. The clue is what you hear: a hum with no start points to the capacitor; a click with no hum points to the contactor or control signal; complete silence points to power or the thermostat.
Safety first
A capacitor can hold a dangerous charge even after the power is off. Do not open the electrical panel of the outdoor unit unless you are qualified. Everything in this guide that goes beyond the easy checks should be done by a technician. Do not keep forcing the system to restart; repeated attempts can burn out the compressor.
What you hear tells you what is wrong
| What you notice | Likely cause |
|---|---|
| Humming from the outdoor unit, fan not turning, no cooling | Failed capacitor or a seized motor |
| Fan runs but the compressor does not | Failed capacitor (dual-run), contactor or compressor |
| A click, then nothing | Contactor problem or tripped overload |
| Total silence outside | No power, tripped breaker or no call for cooling — see outside unit not running |
| Starts, then trips the breaker | Failing compressor or a short — see breaker tripping |
Easy checks first
- Check the thermostat
Set it to Cool and at least three degrees below the room temperature. See thermostat not working.
- Check both breakers and the outdoor disconnect
The AC usually has a double breaker; make sure it is fully on, and that the outdoor disconnect box is in place.
- Replace a dirty filter
A blocked filter can freeze the coil and stop the system. See air conditioner freezing up.
- Allow a few minutes after a shutdown
Many systems have a built-in delay of up to five minutes before the compressor restarts after power returns.
Thermal overload and the waiting period
A compressor that overheated, perhaps from dirty coils or a hot day, can trip its internal thermal overload and refuse to start until it cools. Switch the system off at the thermostat and the breaker and leave it for about an hour. If it then starts normally, the underlying cause is likely overheating from restricted airflow or dirty coils; see AC not cooling enough.
The capacitor
The capacitor gives the motor the jolt it needs to start and keep running. A bulging top, an oily leak or a hum with no start are classic failure signs. It is one of the most common AC failures and a relatively inexpensive part, but swapping one means working with stored high-voltage charge. A technician will test it, and may suggest a hard-start kit for an aging compressor. Our central air not turning on guide also covers capacitor failure.
The contactor
The contactor is the relay switch that sends power to the compressor and fan. Pitted or burnt contacts, or a coil that has failed, mean the click is heard but power never arrives. Replacement is a quick job for a technician.
Low voltage and locked rotor
If the supply voltage is low, or the compressor has seized (a locked rotor), the unit will hum, then trip the overload or the breaker. These are serious faults. A seized compressor is often a reason to consider replacing the whole outdoor unit; read repair or replace your AC before agreeing to an expensive repair on an older system.
When to call an HVAC technician
Anything beyond the thermostat, breaker and filter checks needs a licensed technician. Mention what you heard, when it started and whether the unit tripped the breaker. For ongoing short bursts of running, see AC short cycling.
