What an AC contactor does and why it matters

Your air conditioner's contactor is an electrical switch that turns the compressor and fan on and off. When you set your thermostat to cool, it sends a signal that closes the contactor, allowing electricity to flow to the compressor. If the contactor gets stuck open or fails to close, your AC won't cool — even though the thermostat thinks it's working.

A bad contactor is one of the most common reasons an AC unit runs but produces no cold air. The good news is that you can test whether it's working before you call a technician, and the test takes about five minutes.

Key Takeaways

  • A contactor is a switch that lets electricity reach your AC compressor; if it fails, your AC runs but doesn't cool.
  • You can hear a contactor working — it makes a distinct clicking sound when the thermostat calls for cooling.
  • A multimeter set to resistance mode can tell you whether the contactor's contacts are open or closed, but you must turn off power first.
  • If the contactor clicks but the compressor doesn't start, the problem is likely downstream; if it doesn't click at all, the thermostat or wiring may be at fault.
  • Replacing a contactor costs between $300 and $500 in most cases and is a job for a licensed technician.

Listen for the clicking sound

The simplest test is also the most reliable: turn your thermostat to cool and listen to the outdoor unit. You should hear a distinct click within a few seconds. That click is the contactor closing. If you hear it, the contactor itself is responding to the thermostat signal.

If you don't hear a click at all, the problem is upstream — either the thermostat isn't sending a signal, the wiring between the thermostat and the outdoor unit is broken, or the contactor coil (the electromagnet that pulls the switch closed) has failed. If you hear the click but the compressor doesn't start running, the contactor is closing but something else is preventing the compressor from turning on — usually a capacitor, overload protection, or a refrigerant issue.

Stand near the outdoor unit for at least 30 seconds after adjusting the thermostat. Sometimes there's a delay before the signal reaches the unit. If you still hear nothing, move to the next test.

Locate the contactor in your outdoor unit

To test the contactor with a multimeter, you first need to find it. Turn off power to your AC system at the breaker — do not skip this step. Once power is off, open the outdoor unit's access panel (usually held on with four bolts or a latch). The contactor is a rectangular box about the size of a deck of cards, typically mounted on the side of the unit near the compressor.

It will have two or three large terminals on top (where thick wires connect) and a coil underneath (a smaller cylindrical part with two thin wires). The large terminals are where the electrical contacts are; the coil is what pulls those contacts together when the thermostat calls for cooling.

If you're unsure which part is the contactor, look for the part with the most visible wear — contactors often show burn marks or pitting on the terminals from years of switching current on and off.

Test the contactor with a multimeter

Set your multimeter to the resistance setting (usually marked with an ohm symbol, Ω). With power still off, place one probe on each of the two large terminals on the contactor. A working contactor should show very low resistance — typically less than 1 ohm, or the multimeter may display "0" or "OL" (open line, meaning infinite resistance, which indicates a broken contact).

If the multimeter shows "OL" or a very high number (above 10 ohms), the contactor's contacts are stuck open and the contactor has failed. If it shows near zero, the contacts are closed and the contactor is mechanically sound. This test only tells you whether the contacts can conduct electricity; it doesn't tell you whether the coil is working or whether the contactor will actually close when power is restored.

For a more complete test, turn the power back on (after closing the unit panel), set the thermostat to cool, and use the multimeter's voltage setting to measure the voltage across those same two terminals. You should see 240 volts (or close to it) when the thermostat is calling for cooling. If you see 0 volts, the contactor isn't closing even though it's receiving power, which means the coil has failed.

What to do if the contactor is bad

If your tests show the contactor is stuck open or the coil isn't responding, the contactor needs to be replaced. This is not a repair you should attempt yourself unless you are a licensed HVAC technician — contactors carry high voltage and improper installation can damage the AC system or create a fire hazard.

Call an HVAC technician and tell them you've already identified the contactor as the likely problem. This can speed up the diagnosis and may lower the service call cost. A new contactor typically costs between $300 and $500 installed, depending on your system and your region. Some warranties cover contactors for five to ten years, so check your paperwork before paying out of pocket.

When the contactor clicks but the AC still doesn't cool

If you hear the contactor click and the compressor starts running but no cold air comes out, the contactor is working correctly. The problem is elsewhere — usually a refrigerant leak, a clogged filter, a failed capacitor, or a compressor that's running but not compressing. These require different tests and are beyond what you can safely check yourself.

If the compressor starts but shuts down after a few seconds, the system's overload protection is kicking in, which usually means the compressor is drawing too much current. This can happen if the refrigerant pressure is wrong, the compressor is failing, or the capacitor is weak. Again, this is a technician's job.

Frequently Asked Questions

Can I test the contactor without turning off power?

No. Testing with a multimeter requires you to place probes on live terminals, which risks electrocution. Always turn off power at the breaker before opening the unit or touching any internal parts. If you're not comfortable working with electricity, call a technician instead.

What does it mean if the contactor clicks multiple times?

A single click when the thermostat calls for cooling is normal. Repeated clicking or chattering usually means the coil voltage is too low, the thermostat is sending an unstable signal, or the contactor is beginning to fail. Have a technician inspect it soon — a failing contactor can damage other components.

How long do contactors usually last?

Most contactors last 10 to 15 years, though some fail earlier if the system cycles on and off frequently or if the electrical supply is unstable. There's no way to predict failure, so replacement is usually reactive rather than preventive.

Is a contactor the same as a capacitor?

No. A capacitor stores electrical charge and helps the compressor and fan start; a contactor is a switch that controls whether electricity reaches them at all. Both can fail independently, and both are common AC problems. They're different parts with different functions.