Capacitor (Run/Start)
A small electrical component that gives AC and heat pump motors their starting kick and smooths their running power. It's the part most murdered by heat waves — and the classic symptom is an outdoor unit humming or clicking while its fan stands still.
The longer answer
Capacitors are inexpensive, wear items, and diagnosable with a meter — the trifecta that makes them the best argument for pre-season maintenance. A weak capacitor still 'works' right up until the first sustained hot stretch asks the motor for everything.
If your indoor blower runs but the outdoor unit hums without the fan spinning: shut the system off and book service. Running a motor against a failed capacitor overheats it — converting a modest repair into a motor replacement.
The Seattle angle
Capacitors weaken measurably before they die, which is why the spring tune-up's capacitor reading prevents so many first-heat-wave failures in Seattle's repair queues.
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