A contactor is a heavy-duty electrical switch (relay) that connects high-voltage power to the outdoor unit when the thermostat calls for cooling. It’s essentially a beefier version of a household light-switch relay, rated for the high current loads of compressors and condenser fans.
How a contactor fails
- Pitted contacts — arcing wears the contact surface, eventually preventing reliable closure
- Burnt coil — the low-voltage coil that energizes the contactor fails
- Stuck closed — contactor welds itself shut; AC runs constantly
- Insect intrusion — ants and wasps love contactors; their bodies prevent contact closure
Contactors are often replaced alongside capacitors on the same service call — they’re both ~$30 parts that take similar time to swap, and they typically fail in the same age range.
The “ant problem” is real in Colorado
We pull dead ants out of contactors regularly. It’s so common that some contactor brands now include a built-in pesticide. If your contactor fails and the cover shows ant trails, ask about ant prevention around the outdoor unit.
Related: Capacitor, Compressor.
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Right Way Mechanical is a family-owned HVAC contractor serving Wheat Ridge, Arvada, west Denver, and the Colorado foothills.