What an AC contactor does and why it matters

An AC contactor is an electrical switch inside your air conditioning unit 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 — the part that actually cools your air. When the temperature reaches your target, the thermostat opens the contactor again, cutting power to save energy.

A bad contactor is one of the most common reasons an AC unit stops cooling even though the thermostat works fine. The switch gets stuck open and won't close, or it closes but won't open again. You can check whether yours is the problem before calling a technician, and the test takes about five minutes.

Key Takeaways

  • A contactor is a switch that turns your compressor on and off; if it fails, your AC won't cool even if the thermostat works.
  • You can hear and see a bad contactor: listen for a clicking sound when the thermostat calls for cooling, and look for burn marks or pitting on the metal contacts inside.
  • The contactor sits in the outdoor unit in a metal box about the size of a deck of cards, with thick wires connected to it.
  • If the contactor is burned or pitted, or if you hear clicking but the compressor doesn't start, the contactor likely needs replacement.
  • Replacement costs between $150 and $300 in most regions, and a technician can install one in under an hour.

Where to find your AC contactor

The contactor is always in your outdoor AC unit, never inside the house. Open the metal panel on the side or front of the outdoor box — you may need a screwdriver to remove it. Inside, you will see a metal component about the size of a deck of cards with thick wires running to it. That is your contactor.

If you are not comfortable opening the panel, you can still listen for the contactor from outside. Stand near the outdoor unit with the thermostat set to cool. A working contactor makes a distinct clicking sound when the thermostat calls for cooling — you should hear it click once as the switch closes. If you hear repeated clicking (click-click-click), the contactor is likely failing and trying to close but not staying closed.

The visual inspection: what to look for

Once you have the panel open, look at the metal contacts on the contactor — these are the small metal surfaces where the electrical connection happens. A healthy contactor has shiny, smooth metal contacts. A failing one shows burn marks, pitting (small holes or craters in the metal), or a dark discolored coating.

Burn marks look like black or brown stains on the metal. Pitting looks like the surface has been eaten away, with small craters visible. Either sign means the contactor is not making a clean electrical connection and should be replaced. Do not try to clean the contacts yourself — the damage is permanent, and cleaning will not restore function.

Testing whether the contactor is actually closing

Set your thermostat to cool and watch the contactor. You should see a small plunger (a metal rod) move inward when the thermostat calls for cooling, and the compressor should start within a few seconds. If you hear the clicking sound but the plunger does not move, or if the plunger moves but the compressor does not start, the contactor is failing.

If the compressor starts and runs normally, and the plunger moves smoothly in and out as you adjust the thermostat, your contactor is working. The problem is somewhere else — possibly the capacitor, the compressor itself, or a refrigerant leak.

When a bad contactor causes repeated clicking

Repeated clicking from the outdoor unit usually means the contactor is trying to close but cannot stay closed. This happens when the contacts are too pitted or burned to hold a steady connection. Each time the thermostat tries to energize the contactor, it clicks closed for a fraction of a second, then springs back open, then tries again — creating the clicking pattern you hear.

This is not a sign to keep trying. Repeated clicking means the contactor needs replacement. Continuing to run the unit in this state can damage other components, like the capacitor or the compressor itself, because the power is cutting in and out instead of flowing steadily.

Cost and next steps if the contactor is bad

A replacement contactor costs between $150 and $300 in most regions, depending on your unit's age and size. Labor to install it usually runs $100 to $200. Most technicians can replace a contactor in under an hour, so the total bill is typically $250 to $500.

If you have confirmed the contactor is burned, pitted, or clicking repeatedly, call an AC technician to replace it. Bring a photo of the burned or pitted contacts if you took one — it will help the technician confirm the diagnosis and order the right part. If you are not certain whether the contactor is the problem, a technician can test it with a multimeter to be sure before you pay for replacement.

Frequently Asked Questions

Can I replace the contactor myself?

You can if you are comfortable working with electrical components and have experience with AC units. The contactor itself is not expensive, but the work involves high-voltage wiring. Most people hire a technician because a mistake can damage the unit or create a safety hazard. If you decide to do it, turn off power to the unit at the breaker first.

What is the difference between a contactor and a capacitor?

A contactor is a switch that turns power on and off. A capacitor is a small cylindrical component that stores electrical energy and helps the compressor start. Both can fail, and both are common reasons an AC unit stops working. A bad capacitor usually causes humming without the compressor starting; a bad contactor causes clicking or no response at all.

Will a bad contactor damage my compressor?

Yes, if left running. Repeated clicking means power is cutting in and out, which stresses the compressor and can shorten its life. If you hear clicking, stop running the unit and call a technician. Replacing the contactor now is much cheaper than replacing the compressor later.

How long does a contactor usually last?

Most contactors last 10 to 15 years, though some fail sooner if the unit runs constantly or in very hot climates. There is no way to predict failure, so the best approach is to listen and look when your AC stops cooling, rather than replacing it on a schedule.