AC Contactor Symptoms: What It Is and What Fails

When the thermostat asks for cooling, it does not switch the compressor on directly. It energizes a small coil on the contactor. That coil pulls a set of heavy contacts closed, and those contacts feed line power to the compressor and the condenser fan. When the call ends, a spring opens the contacts. It is a switch that lives outdoors, in the heat, and it pits over time.

What failure looks like

• The thermostat is calling, the indoor blower may be running, and the outdoor unit is silent. A contactor that is not pulling in is one cause. So is a failed capacitor, a safety switch, or no low-voltage signal.
• The outdoor unit hums. Burned contacts can pass a partial connection. A bad capacitor does this too.
• The outdoor unit runs and will not stop. A contactor welded shut keeps the compressor powered after the thermostat is satisfied. Shut the system off at the breaker or the disconnect and leave it off until it is repaired. A compressor that never rests can destroy itself.

What not to do at the panel

Do not take the service panel off and press the contactor plunger with a screwdriver or a stick. You are standing at line voltage, and if the compressor starts you are in the fan blade's path. Do not sand the contacts. Do not swap in a contactor with a different coil voltage or a different amp rating because it fit the screws.

Insects and ants nest in contactors and hold them mechanically. That is still a reason to kill power and have it cleaned or replaced properly, not a reason to poke around a live unit.

An outdoor unit that will not start, or will not stop, is AC repair. The contactor is tested with the capacitor and the motors. It is a part, not a new system.

Common questions