What is a contactor?
Updated August 2026
A contactor is an electrically operated switch inside the outdoor unit that connects 240-volt power to the compressor and condenser fan whenever the thermostat calls for cooling. The thermostat itself sends only a 24-volt signal, nowhere near enough to turn a compressor. That small signal energizes a coil in the contactor, the coil pulls a plunger down, two silver contacts close, and the high-voltage circuit completes. The clunk you hear outside a second before the fan starts is this part doing its job.
Where it is
Pull the screwed service cover off the side of the outdoor condenser and it is the black block about 2 inches wide sitting next to the capacitor, with two heavy wires landing on top, two heavy wires below, and two thin control wires on the coil terminals. The movable metal bar across the middle is the plunger. Before that cover comes off, pull the outdoor disconnect and switch off the breaker, because the line-side lugs stay live any time the disconnect is seated, and treat the capacitor beside it as charged. Heat pumps use the same part in the same spot.
What it does
This switch opens and closes thousands of times a season, and every break draws a small arc that burns metal off the contact faces. It ends one of two ways. Pitted, blackened contacts stop passing full voltage, so the unit buzzes and refuses to start; tapping the plunger with an insulated screwdriver often kicks it to life, which counts as a diagnosis rather than a repair. Or the contacts weld shut, and now the compressor runs whether the thermostat asked or not, freezing the indoor coil and adding real money to the July bill. Ants are the other classic killer: they crawl toward the warmth, get crushed under the plunger, and hold the contacts apart. Expect 5 to 10 years from one.
Maintenance
Ask for a look at the contact faces during the annual spring tune-up ($100–250) and replace the part at the first sign of pitting or a burnt smell, rather than after it strands you on a hot Saturday. Because the contactor and the capacitor sit side by side and wear out at similar ages, many technicians swap both in one visit for $250–450 total, cheaper than two separate trips. Clean condenser coils help as well, since lower head pressure means lower amp draw across those contacts. See how often to clean condenser coils and what skipping it costs.
What it costs to replace
A replacement contactor runs $10–30 for the part, $150–350 installed, or $250–450 when the capacitor is changed at the same visit.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
What is the difference between a contactor and a relay?
Size and duty. Both are switches thrown by an electromagnet, but a relay handles small loads, a few amps, on control circuits. A contactor is built for high current, typically 30 to 40 amps at 240 volts in a house, with heavier contacts and an arc-resistant design. In the outdoor unit the contactor feeds the compressor and fan directly, while relays inside the furnace or air handler manage low-voltage logic. Order one by amp rating and pole count, not by brand.
Why does my AC hum outside but never start?
Two suspects share that symptom. A failed capacitor leaves the motors without their starting boost, so the fan sits still while the unit hums. A pitted contactor never passes full voltage, so you hear the coil pull in and nothing follows. A technician separates them in about two minutes with a meter, and both parts are cheap: $150–400 for a capacitor, $150–350 for a contactor. Leaving either to hum overnight risks the compressor, so shut the system off at the thermostat while you wait for the appointment.
Can I replace an AC contactor myself?
It is a 20 minute job for someone disciplined about power. Pull the disconnect, turn off the breaker, discharge the capacitor, photograph every wire, then move connections one at a time to the matching terminals on the new part. Buy the same voltage, amperage, and pole count as the original. What makes people regret it: crossing line and load sides, or forgetting that the top lugs stay energized. If any of that is unfamiliar, $150–350 for a pro is a fair trade.