How to tell if a contactor is broken?

Determining whether a residential air conditioning contactor is broken involves observing equipment startup failures, conducting visual inspections, and performing electrical tests. A primary sign of a broken contactor is that your outdoor condensing unit emits a loud buzzing sound from the electromagnetic coil when the thermostat calls for cooling, but the compressor and fan fail to engage because the mechanical plunger fails to push the contact points closed. Visually, a broken contactor will exhibit heavily charred copper points, melted plastic housing, or a rusted, stuck plunger. You can confirm electrical failure using a digital multimeter to test continuity across the terminals. You can troubleshoot safely by turning off power at the disconnect box. You can verify that the contactor is broken when multimeter testing confirms an open electromagnetic coil or high resistance across closed contacts, requiring immediate replacement.