What a contactor does and why it matters
A contactor is an electrical switch inside your air conditioner, heat pump, or furnace that turns the outdoor unit on and off. When your thermostat tells the system to run, it sends a signal to the contactor, which closes and allows electricity to flow to the compressor and fan. If the contactor fails, your outdoor unit won't start even though your indoor thermostat is calling for cooling or heating.
The contactor is one of the most common parts to fail in an HVAC system because it switches on and off hundreds of times per year. Each time it switches, the electrical contacts wear down slightly. After several years, the contacts become pitted or corroded and stop making a solid connection.
You can check whether your contactor is working by listening for a click, feeling for vibration, and looking for visible damage. A failed contactor usually means your outdoor unit won't run at all, even though you hear the indoor fan and the thermostat is set correctly.
Key Takeaways
- A contactor is an electrical switch that turns your outdoor AC or heat pump unit on when the thermostat calls for cooling or heating.
- You can hear a clicking sound at the outdoor unit when the contactor closes, and feel a slight vibration if it is working.
- A burned smell, visible pitting on the contacts, or complete silence from the outdoor unit usually means the contactor has failed.
- If the contactor is bad, the outdoor unit will not run, but the indoor thermostat and blower will still work normally.
- Replacing a contactor costs between $150 and $400 in labor and parts, depending on the brand and your location.
Listen for a click when the system starts
The easiest way to check a contactor is to stand near your outdoor unit and listen when you turn on the air conditioner or heat pump from your thermostat. A working contactor makes a distinct clicking or buzzing sound as it closes and engages the electrical circuit. The click happens in the first second or two after you set the thermostat to cool or heat.
If you hear the click, the contactor is closing and sending power to the compressor. If you hear nothing at all from the outdoor unit — no click, no hum, no fan noise — the contactor may be stuck open or the problem lies elsewhere in the electrical circuit.
On a hot day, turn the thermostat down several degrees below the current temperature to force the system to call for cooling. Wait a few seconds and listen at the outdoor unit. A healthy system will click within two seconds.
Feel for vibration in the outdoor unit
After you hear the click, place your hand gently on the side of the outdoor unit (not on any moving fan blades). A working system will vibrate slightly as the compressor starts running. The vibration should feel steady and continuous once the unit is running.
If you hear a click but feel no vibration, the contactor may be closing but not delivering full power to the compressor. This can happen if the contacts are corroded or partially burned. The compressor may try to start and then shut down when ready, which is why you hear the click but the unit doesn't actually run.
If you feel vibration but the system is not cooling or heating, the problem is likely not the contactor — it may be a refrigerant leak, a blocked filter, or a problem with the compressor itself.
Look for visible damage to the contactor
If you are comfortable opening the outdoor unit's electrical panel (turn off power at the breaker first), you can inspect the contactor directly. The contactor is a rectangular box about the size of a deck of cards, usually mounted on the side of the unit near the top. It has two or three large terminals on top where thick wires connect.
Look at the contacts inside the contactor — they are the small metal points that touch each other when the switch closes. Burned contacts look black, pitted, or corroded. If the contacts are heavily discolored or pitted, the contactor is failing and should be replaced. A light gray or tan color is normal wear.
You may also smell a burned or acrid odor coming from the contactor if it is failing. This smell is the insulation on the coil burning as the contactor struggles to switch. Do not ignore this smell — a failing contactor can overheat and create a fire risk.
Check whether the outdoor unit runs at all
The most obvious sign of a bad contactor is that the outdoor unit does not run, even though the thermostat is set to cool or heat and the indoor blower is running. If the indoor fan works but the outdoor unit is completely silent and still, the contactor is a likely culprit.
To rule out other problems, check that the outdoor unit's disconnect switch (a small box near the unit) is in the on position. Check that the breaker for the outdoor unit has not tripped. If both are on and the unit still will not run, the contactor is probably the problem.
A contactor can also fail partially, causing the outdoor unit to start and stop repeatedly every few seconds. This is called short cycling and is another sign the contactor needs replacement.
When to call a technician
If you have confirmed that the contactor is not clicking, the outdoor unit is not vibrating, or you see burned contacts, the contactor needs to be replaced. This is not a repair you should attempt yourself unless you are a licensed electrician, because the outdoor unit carries high-voltage electricity even when the breaker is off.
A technician will turn off power to the unit, remove the old contactor, and install a new one. The job usually takes 30 minutes to an hour. Most contractors charge between $150 and $400 for labor and parts, depending on the brand of your unit and your location.
If the contactor failed early — within five years of installation — ask the technician whether the problem is a power surge, a wiring issue, or straightforward normal wear. A second failure in a short time may point to an underlying electrical problem that should be fixed to protect the new contactor.
How to prevent contactor failure
You cannot stop a contactor from wearing out eventually, but you can slow the process. Keep your outdoor unit clean and free of debris, because dirt and dust make the contactor work harder. Replace your air filter every one to three months so the system does not strain.
Power surges from lightning or utility problems can burn out a contactor when ready. If you live in an area with frequent thunderstorms, a whole-home surge protector may be worth the investment. It protects not just the contactor but also the compressor and other electrical components.
Have your system serviced once a year by a technician. During a service call, the technician will inspect the contactor for early signs of wear and can replace it before it fails completely, saving you from a breakdown on the hottest day of the year.
Frequently Asked Questions
Can a contactor fail without making any noise?
Yes. A contactor can fail silently if the contacts are so corroded that they never close at all. You will hear nothing from the outdoor unit — no click, no hum, no fan noise. The system will not run even though the thermostat is calling for cooling or heating.
Is it safe to touch the outdoor unit while it is running?
You can touch the metal casing of the outdoor unit while it is running, but avoid the fan blades and any exposed wires. Do not open the electrical panel or touch any terminals or contacts while the power is on. Always turn off the breaker before opening the unit.
What does a burned contactor smell like?
A burned contactor smells acrid or like melted plastic — a sharp, unpleasant odor that is noticeably different from normal air conditioner smells. If you smell this, turn off the system and call a technician. Do not run the system again until the contactor is replaced.
How long does a contactor usually last?
Most contactors last 10 to 15 years with normal use. In areas with frequent power surges or in systems that run constantly, a contactor may fail after 5 to 7 years. Proper maintenance and a surge protector can extend the life of the contactor.
Can I replace the contactor myself?
Replacing a contactor requires working with high-voltage electricity and is not a job for a homeowner. The outdoor unit can hold a charge even after the breaker is off. Hire a licensed HVAC technician to replace the contactor safely.