How do you know if a capacitor has failed?

Determining whether a residential HVAC capacitor has failed involves observing specific motor startup symptoms, listening for unusual noises, and performing electrical tests. A primary sign of a failed capacitor is that your outdoor air conditioning compressor or indoor blower motor emits a loud, rhythmic humming or buzzing sound but refuses to spin up, eventually tripping the thermal overload switch or circuit breaker. Another sign is intermittent fan operation or complete failure of the cooling system to engage when the thermostat calls for air. You can confirm failure by inspecting the unit for a bulging top casing or by testing microfarad output with a digital multimeter. You can verify that the capacitor has failed when multimeter readings fall significantly below the manufacturer's rated microfarad specifications, requiring a new replacement part.