A capacitor is an electrical storage device that supplies a brief burst of current to start (and in some cases run) the compressor and the outdoor condenser fan. Most outdoor units have one dual-run capacitor that serves both motors.
Capacitors are a very common failure point — they degrade over time and especially fail after lightning strikes or power surges. Symptoms include the outdoor unit humming but not starting, the fan running while the compressor doesn't (or vice versa), or repeated short-cycling.
Replacement is usually a 15-minute job and an inexpensive part. It's one of the most common service calls we run in the Lowcountry, particularly after summer thunderstorms.
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