Bad AC capacitor: symptoms and how a tech tests it

The motors in your outdoor unit need help to start and to run smoothly. A capacitor stores a small electrical charge and releases it to give the compressor and the fan motor that push. Most residential units use one dual run capacitor, a metal can with three terminals that serves both motors. Some systems add a separate start capacitor for the compressor, and the indoor blower motor often has its own small capacitor.

When the capacitor weakens, the motors start slowly and run hot. When it fails completely, they hum and sit still. Because one dual capacitor has two sections, one side can fail alone. That is why you sometimes see the fan running with no compressor, or the compressor running with no fan.

1. Humming or buzzing outdoor unit that does not start.
2. Outdoor fan not spinning while the unit hums, or spinning slowly.
3. Warm air from the vents with the indoor fan running normally.
4. Hard starts. The unit shudders, takes several tries, or dims the lights more than it used to.
5. Shutting off after a short run as the compressor overheats and trips its internal protector.
6. A tripped breaker at startup.
7. A swollen or leaking capacitor. A technician will see a domed top or oil on the can. This is visible only with the panel off.

None of these symptoms belongs to the capacitor alone. A worn contactor, a seized fan motor and a failing compressor can look the same from the yard. The capacitor is simply the most likely and the least expensive of them.

The test takes a few minutes. The technician shuts off power at the disconnect, confirms it is off with a meter, and discharges the capacitor safely. They disconnect the leads and measure capacitance with a meter that has a microfarad setting. The reading is compared with the rating and tolerance printed on the label. A reading below the allowed range means the part is weak and should be replaced, even if the unit still starts.

Some technicians also test under load with the system running, using voltage and current readings. Either method is valid. A visual check alone is not enough, because many failed capacitors look perfect.

The replacement must match the original microfarad rating and meet or exceed its voltage rating. Ask the technician to show you the reading on the old part. The capacitor replacement cost guide covers what you are paying for on that visit.

Heat is the main enemy. The capacitor sits in a metal box in the sun next to a hot compressor, and summer is when it works hardest. Power surges, lightning, voltage swings and plain age finish the job. A capacitor that fails repeatedly on the same unit is a clue that a motor is drawing too much current or the unit is running too hot, and that deserves a closer look.

A capacitor is a routine part swap and never a reason to replace an air conditioner. Be skeptical if a failed capacitor turns into a system replacement pitch without further test results. The exception is when the new capacitor does not fix the problem because the compressor behind it has failed. The AC repair overview covers what the next steps look like in that case.