A contactor is an electromechanical relay inside the outdoor condensing unit. When the thermostat calls for cooling (or heating in heat pump mode), it sends a 24-volt signal to the contactor's coil, which pulls the contacts together and completes the 240-volt circuit to the compressor and fan.
Contactors wear out from cycle count — every start and stop arcs the contacts slightly. Over years (and especially in salt-air environments), the contacts pit and corrode, eventually failing to complete the circuit or sticking closed and refusing to release.
Like capacitors, contactor replacement is a quick, inexpensive service call. A failing contactor is one of the most common reasons "the outdoor unit won't start."
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