In short
An AC contactor is the switch inside the outdoor unit that the thermostat controls. A 24-volt signal closes its contacts and feeds 240 volts to the compressor and fan; when the signal stops, a spring opens them. Pitted contacts cause no-starts and welded contacts keep the unit running after the house is cool. Replacement runs $150–$350 in the GTA.
Every time your thermostat asks for cooling, one small part outside does the actual switching. What is an AC contactor? It is a spring-loaded relay about the size of a deck of cards, mounted in the electrical compartment of the condenser beside the capacitor. Thin low-voltage wire from the house lands on its coil, and the heavy power wires from the disconnect land on its contacts.
What an AC contactor does each time the thermostat calls
The thermostat sends 24 volts down the Y wire, through the furnace control board and out to the condenser. That energizes the contactor's coil, which becomes an electromagnet and pulls a plunger down, closing the contacts and passing 240 volts to the compressor and condenser fan. When the house reaches setpoint the 24 volts drops, a spring lifts the plunger and the unit stops. The 24 volts comes from the transformer in the furnace, so a contactor problem and a furnace low-voltage problem can look alike from the hallway.
Residential contactors come in two styles. A single-pole contactor switches one of the two power legs and leaves the other connected through a solid bar, so parts of the unit stay energized whenever the breaker is on. A two-pole contactor breaks both legs. The replacement has to match the original's pole count, coil voltage and amp rating.
How an AC contactor fails, and why it happens in spring here
A condenser in southern Ontario sits idle for roughly seven months of the year. Over that stretch moisture works on the contact faces, and ants and earwigs move into the dry compartment and get crushed between the contacts on the first call for cooling. That is why contactor calls cluster at spring start-up across Burlington, Oakville and Hamilton. The failure modes are distinct.
- Pitted or burnt contacts: every start draws an arc, the faces erode, and eventually they pass too little current. The unit hums, starts late or does not start.
- Welded contacts: the faces fuse together and the outdoor unit keeps running after the thermostat is satisfied, sometimes with the indoor blower off. Turn off the breaker, because the indoor coil can freeze solid this way.
- Chattering or buzzing: a weak 24-volt supply, a failing coil or debris under the plunger makes the contactor rattle in and out, which hammers the compressor.
- Open coil: the winding breaks, nothing pulls in, and the outdoor unit is silent while the indoor fan blows room-temperature air.
- Shorted coil: the winding shorts and blows the low-voltage fuse on the furnace control board, so the thermostat may go blank or the whole system stops.
A shorted coil is the one that fools people, because the symptom shows up at the furnace. If the small blade fuse on the furnace board blows again as soon as cooling is called, the fault is often outside: the contactor coil, or the thermostat wire running to it after a trimmer has nicked it.
What AC contactor replacement costs in Ontario
- Service call and diagnosis: $90–$150, applied to the repair when you proceed.
- Contactor replacement, installed: $150–$350.
A contactor rarely fails alone. Burnt contacts starve the compressor of voltage, which stresses the AC capacitor, and a weak capacitor makes the contacts arc harder. A proper repair includes a microfarad reading on the capacitor and an amp draw on the compressor. The job itself is described on our AC contactor replacement page, and general price bands are on the pricing page.
When to call for AC repair and what to do first
If the outdoor unit will not stop, switch off its breaker at the panel. If it will not start, set the thermostat to off so the compressor is not left humming against a bad connection. Do not press the contactor plunger with a screwdriver to test it: the terminals beside it are live at 240 volts. Book AC repair or reach us through the contact page, and tell us whether the furnace fan still runs, because that narrows the fault before we arrive.
Questions people ask about AC contactor
Is it normal for an AC contactor to buzz?
A faint hum from the coil is normal. A loud buzz or a rapid clicking is not: it means the plunger is not seating, usually from low control voltage, a weak coil, or dirt and insects in the mechanism. Chattering makes and breaks power to the compressor many times a second and shortens its life, so have it looked at before the next heat wave.
How long does an AC contactor last?
It depends on how many starts it sees and what it sits in. A unit that short-cycles wears its contacts far faster than one with long, steady runs, and a condenser beside a garden bed collects more insects. Contact condition is checked at every tune-up, and visibly pitted or blackened faces are replaced before they strand you in July.
Can ants really stop an air conditioner from working?
Yes. Ants are drawn into the electrical compartment and get caught between the contact faces when the contactor closes. Enough of them form an insulating layer, the contacts no longer pass power, and the unit will not start. The fix is a new contactor and a cleaned-out compartment; dealing with the nest near the pad keeps them from coming back.