A bad AC capacitor is one of the most common reasons an air conditioner stops working on a hot day. The classic sign is an outdoor unit that hums but whose fan doesn’t spin. The good news: it’s usually one of the cheaper AC repairs. The important caveat: capacitors store a high-voltage charge even when the power is off, so replacing one is a job for a licensed technician unless you have real electrical training.
What an AC capacitor does
Motors need a strong push to start turning. The capacitor stores electrical energy and releases it as a jolt that gets the compressor and the outdoor fan motor started, and then helps keep them running efficiently.
Most central air conditioners use a dual run capacitor, a single metal cylinder that serves both the compressor and the fan motor. Some systems also have a separate start capacitor or a “hard start kit” that gives the compressor an extra boost.
Capacitors are rated in microfarads (µF or MFD) and voltage. A typical dual capacitor label might read something like 45/5 MFD, 440 VAC: 45 for the compressor, 5 for the fan.
Symptoms of a bad AC capacitor
- The outdoor unit hums but the fan doesn’t spin. This is the most common sign.
- The fan spins slowly, or starts only if pushed.
- The AC blows warm air while the indoor blower runs normally.
- The outdoor unit clicks or tries to start and then stops.
- The AC shuts off on its own or takes a long time to start.
- Rising energy bills as a weak capacitor makes motors work harder.
- A bulging, swollen or leaking capacitor (visible only to someone opening the electrical panel, which should be a technician).
What causes capacitors to fail
- Heat. Capacitors degrade faster in very hot climates and in units sitting in direct afternoon sun.
- Age. Many last 10 to 20 years, but cheaper capacitors can fail in a few years.
- Power surges and lightning.
- Wrong rating. A replacement with the wrong microfarad or voltage rating can fail early or damage motors.
- A struggling motor that draws more current than normal.
What you can safely do
- Turn the AC off at the thermostat. If the compressor is trying and failing to start, letting it keep trying can overheat it.
- Check the obvious first: the thermostat setting, the breaker and the outdoor disconnect. Our AC not cooling guide walks through these quick checks.
- Write down the model number of your outdoor unit (on the data plate on its side). It helps the technician arrive with the right part.
- Call a licensed HVAC technician.
Why you shouldn’t replace it yourself
You’ll find many videos showing DIY capacitor swaps. Here’s why we recommend a professional for most homeowners:
- Shock hazard. A capacitor can hold a dangerous charge long after the power is turned off. It must be discharged correctly with insulated tools before it’s touched.
- Live wiring. The outdoor unit’s electrical compartment carries 240 volts.
- Exact matching matters. The replacement must match the microfarad rating (usually within a few percent) and meet or exceed the voltage rating.
- Wiring errors can damage the compressor, the most expensive part of the system.
- Misdiagnosis. The same symptoms can come from a failed contactor, a seized fan motor or a compressor problem. A technician tests each part with a meter.
How much does an AC capacitor replacement cost?
| Item | Typical cost* |
|---|---|
| Capacitor part alone | $15 to $60 |
| Professional replacement (part, labor and service call) | $150 to $450 |
| Hard start kit, installed | $200 to $500 |
| After-hours or weekend emergency service | Often $100 to $300 extra |
*Typical U.S. ranges; prices vary by region and contractor. Be cautious with quotes far above these ranges and ask what’s included. See our AC repair cost guide for other common repairs.
How to make capacitors last longer
- Keep the outdoor unit clean and well ventilated. See how to clean AC condenser coils.
- Consider shade for the unit, as long as airflow isn’t blocked.
- Install a surge protector on the outdoor unit if you’re in a lightning-prone area.
- Have a technician test capacitor values during your annual tune-up. A weakening capacitor can be replaced before it fails in the middle of a heat wave.
Frequently asked questions
How long do AC capacitors last?
Often 10 to 20 years, but heat, power surges and lower-quality parts can shorten that to a few years.
Can a bad capacitor damage my compressor?
Yes. A compressor that repeatedly struggles to start overheats and wears faster. Turn the system off and get the capacitor replaced promptly.
Can I run my AC with a bad capacitor?
You shouldn’t. The compressor or fan motor may overheat trying to start, which can turn a small repair into a big one.
Is a humming outdoor unit always a bad capacitor?
No. It’s the most common cause, but a failed contactor, a seized fan motor or a compressor problem can cause similar symptoms. A technician can test each one. See also outdoor AC fan not spinning.
Disclaimer: this guide is for general information only. AC capacitors and outdoor units carry high voltage that can cause serious injury. Contact a licensed HVAC professional for diagnosis and repair.