Why an AC Capacitor Fails and What the Symptoms Look Like

If there is one part of a residential air conditioner that a Los Angeles homeowner is likely to hear about eventually, it is the capacitor. It is small, it is inexpensive as parts go, and when it fails it produces symptoms dramatic enough to make people fear the worst. Understanding it takes the fear out of the phone call.

What a capacitor does

The compressor and the fans in your system are electric motors, and single-phase household motors cannot start themselves cleanly. The capacitor stores a charge and releases it at the moment of starting, giving the motor the push it needs to get turning, and then it helps keep the motor running efficiently. In the outdoor unit there is usually one dual capacitor serving both the compressor and the condenser fan; the indoor blower has its own on many older systems.

When the capacitor is healthy, the outdoor unit starts with a confident hum and the fan spins up at once. When it is weak, the motor tries to start, cannot, and either sits there humming or trips its own overload protection. When it has failed outright, nothing starts at all, or only the fan does.

Why they fail

Capacitors are rated for a working temperature, and the inside of an outdoor unit sitting in a Woodland Hills driveway in August is a hostile place. Heat is the main killer: the electrolyte inside dries out, the capacitance drifts downward, and one day it is no longer enough. Age does the same job more slowly, so a unit that has run through many summers is living on borrowed time. Voltage spikes from power surges can finish one off in a moment, and a compressor that has begun drawing more current than it should stresses the capacitor every time it starts.

Two things follow. First, a capacitor failing in the hottest week of the year is not bad luck; it is the expected time. Second, a capacitor that failed early may be a symptom rather than the whole problem, which is why the technician checks what the compressor is drawing before declaring the job done.

What you will notice

The symptoms depend on how far gone it is:

• A hum from the outdoor unit with the fan not turning. The classic sign. The compressor may also be trying and failing, which you hear as a hum that stops after a few seconds and starts again.
• Warm air indoors while the outdoor unit runs poorly. The indoor blower is fine, but no heat is being moved outside.
• The system starts and stops. A weak capacitor lets the compressor start, struggle and drop out, so the AC short-cycles. The short cycling article covers what that does to the compressor over time.
• A breaker that trips at start-up. A motor trying to start without its capacitor draws a heavy current spike, and the breaker does what breakers do. Reset it once; if it trips again, leave it off.
• A clicking outdoor unit. The contactor pulls in, nothing starts, it releases, and the cycle repeats.
• A bulged or leaking capacitor, if it is visible through the vents of the unit. Do not open the unit to look.

If the system will not start at all, the AC won’t turn on guide walks through the safe checks, including the thermostat and the breaker, that rule out the even simpler causes first.

Why this is not a homeowner job

Capacitors store a charge even when the power is off, and a charged capacitor can deliver a painful and dangerous shock. Testing one properly means disconnecting the power, discharging the capacitor safely, and measuring it against its rating, then measuring the current the compressor and fan actually draw to see whether the capacitor was the cause or a casualty. That last step is the one people skip when they buy a part online, and it is the reason a replacement capacitor sometimes fails again within a month: the compressor was the real problem.

The safe homeowner response to any of the symptoms above is to switch the system off at the thermostat, so the compressor stops trying to start, and to call.

What the technician does

The visit follows a short sequence. Power off, then a visual inspection of the capacitor, contactor and wiring for heat damage. A measurement of the capacitor against its rating. A test of the compressor and fan windings. Then power on under supervision, with a meter on the compressor to see what it draws on start-up and while running. A capacitor that reads low with a compressor that draws normal current is a straightforward replacement. A compressor that draws far too much current even with a good capacitor is a different conversation, and you hear it before anything is fitted.

Because capacitors are common parts, the replacement is usually on the van. The diagnostic visit page explains how the visit runs, and the pricing page explains how the diagnostic visit fee is separated from the repair price you approve.

Can you prevent it?

Not entirely; heat and time win in the end. But a clean outdoor coil and clear space around the unit keep the compressor running cooler and drawing less current, which is easier on the capacitor. And a system that starts with a hesitant hum in June is telling you something; having it looked at before the heat wave beats having it fail during one. If your breaker has started tripping when the AC starts, read why an AC keeps tripping the breaker before resetting it again.