AC Compressor Won't Start? What to Check, in Order

What you're seeing

• The outdoor fan spins but the unit isn't cooling.
• A humming sound from the outdoor unit, then it clicks off.
• Nothing runs outside at all while the indoor blower keeps going.
• The breaker trips when the compressor tries to start.

How to track it down

Work through these in order, or follow along in the video above. Each step rules something out before you spend money on parts.

1. Check power at the unit

   Confirm line voltage at the disconnect and check the fuses or breaker. A single blown fuse can leave you with a fan that runs and a compressor that doesn't.

2. Make sure the contactor pulls in

   With a call for cooling there should be 24 volts at the contactor coil. If the coil has voltage but the contacts don't close, or the contacts are pitted and burned, the contactor is the problem.

3. Test the capacitor

   Discharge it safely first, then check microfarads against the rating on the label. A weak or failed run capacitor is the most common reason a compressor hums and won't start.

4. Consider the overload

   A compressor that's been trying to start will be hot, and its internal overload may be open. Give it time to cool before calling it dead.

5. Check windings and try a hard start kit

   Measure resistance across common, start, and run, and check for a short to ground. If the windings are good but it struggles to start, a hard start kit can get an older compressor going.

If you're a homeowner

Capacitors hold a charge even with the power off, and the outdoor unit runs on high voltage. If you're not trained, stop at checking your breaker and thermostat and call a tech. Don't keep resetting a breaker that trips, since that can damage the compressor.

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