What an AC capacitor does and why it fails

An air conditioning capacitor stores electrical energy and releases it to start the compressor and fan motor. Over time — usually five to ten years — the capacitor loses its ability to hold a charge. When it fails, the compressor or fan may hum but not start, or the system may not cool at all. A failed capacitor is one of the most common AC repairs.

You can replace a capacitor yourself if you are comfortable working with electrical components and follow safety steps exactly. The job takes 15 to 30 minutes and costs $15 to $50 for the part, versus $200 to $400 if you call a technician. However, if you are unsure at any point, stop and call a professional — capacitors hold a charge even after power is off, and a shock can cause serious injury.

Key Takeaways

  • Turn off power at the breaker and wait five minutes before touching any part of the unit, because capacitors retain charge even when the system is off.
  • Take a photo of the wire connections before you disconnect anything, so you know exactly where each wire goes back on the new capacitor.
  • Buy a replacement capacitor with the exact same microfarad (µF) rating and voltage as the one you are removing — the rating is printed on the side of the old capacitor.
  • Discharge the old capacitor using an insulated screwdriver across the terminals before you touch it, even though power is off.
  • If the capacitor is mounted in a clip, slide it out; if it is soldered, you will need a desoldering tool and should consider calling a technician instead.

Turning off power and waiting for discharge

Locate your AC breaker in the main electrical panel and switch it to the off position. Wait five full minutes. This time allows any residual charge in the capacitor to bleed away through the system, though capacitors can retain a dangerous charge even after extended downtime.

Do not rely on the breaker switch alone. After the five-minute wait, use a multimeter set to DC voltage to test the capacitor terminals. If the meter reads zero, the capacitor is safe to touch. If it reads any voltage, wait another five minutes and test again.

Identifying the capacitor and its specifications

The capacitor is a cylindrical or oval component mounted near the compressor or fan motor, usually held in a plastic clip or bracket. It has two or three terminals on top — most residential units have two. The side of the capacitor is printed with its rating in microfarads (µF) and voltage (V), for example "45 µF 370V" or "5 µF 250V".

Write down or photograph this rating. You must buy a replacement with the exact same microfarad and voltage numbers. If you install a capacitor with the wrong rating, the motor will not start or the system will fail. Many hardware stores and online retailers let you search by microfarad and voltage, or you can bring the old capacitor with you to match it.

Discharging the capacitor safely

Even with power off, a capacitor can hold enough charge to cause a painful shock or burn. Discharge it by taking an insulated screwdriver — one with a plastic handle — and touching the metal shaft across both terminals at the same time. You may see a small spark; this is normal and means the capacitor has released its charge.

If the capacitor has three terminals (common on dual-capacitor units), discharge across each pair: terminal 1 to terminal 2, terminal 1 to terminal 3, and terminal 2 to terminal 3. After discharging, wait 30 seconds before handling the capacitor.

Disconnecting wires and removing the old capacitor

Before you disconnect anything, take a clear photo of the wire connections. Label each wire or draw a diagram showing which wire connects to which terminal. Capacitor terminals are usually labeled C (common), H (herm, for compressor), and F (fan), or they may be numbered 1, 2, and 3.

Using needle-nose pliers, gently pull each wire terminal off the capacitor terminal. Do not yank — the connection should slide off with steady, even pressure. If a wire is stuck, wiggle it side to side as you pull. Once all wires are free, check whether the capacitor is held in a clip or bracket. If it is clipped, slide it out. If it is soldered to a circuit board, you will need a desoldering tool (a solder sucker or desoldering braid) — if you do not have one, this is the point to call a technician.

Installing the new capacitor

Slide the new capacitor into the clip or bracket in the same position as the old one. Make sure it sits flat and find. Then reconnect the wires to the terminals, using your photo or diagram as a guide. Push each wire terminal onto its terminal firmly until you hear or feel a click. The connection should be snug enough that you cannot pull it off by hand without using pliers.

Double-check that each wire is on the correct terminal. A wire on the wrong terminal will prevent the system from starting or may damage the motor. If you are uncertain, compare your photo to the new capacitor and verify each connection before moving forward.

Testing the system and when to call a technician

Turn the breaker back on and set your thermostat to cool. The compressor and fan should start within 30 seconds. If they do not, turn the breaker off when ready, wait five minutes, and check your wire connections. A common mistake is reversing two wires or placing a wire on the wrong terminal.

If the system still does not start after you have checked the connections twice, or if you hear a burning smell or see smoke, turn off the breaker and call a technician. Do not attempt further troubleshooting. A capacitor that fails when ready after installation may indicate a problem with the motor itself, which requires professional diagnosis.

Frequently Asked Questions

Can I replace a capacitor while the system is running?

No. Always turn off the breaker and wait five minutes before touching any part of the unit. A running system can deliver a lethal shock, and the capacitor will be fully charged.

What if I cannot find a capacitor with the exact same microfarad rating?

Do not substitute. The microfarad rating must match exactly. If your local store does not have it, order online from an HVAC supply retailer or a general parts site. A wrong rating will damage the motor.

Is it safe to discharge a capacitor by touching the terminals with my bare hands?

No. Always use an insulated screwdriver. A direct touch can cause a serious shock or burn. The insulation on the screwdriver handle protects you if the capacitor still holds a charge.

What does it mean if the capacitor is bulging or leaking?

A bulging or leaking capacitor has failed internally and must be replaced. Do not attempt to repair it. A leaking capacitor may have corrosive fluid on it, so wear gloves when handling it and wash your hands afterward.

How do I know if the new capacitor is working?

If the compressor and fan start within 30 seconds of turning on the system, the capacitor is working. If the system runs normally and cools your home, the installation was successful. If the system hums but does not start, the capacitor may be bad or a wire may be on the wrong terminal.