What a capacitor does and when to replace it

A capacitor is a small cylindrical part inside your AC unit that stores electrical energy and helps start the compressor and fan motor. When a capacitor fails, your air conditioner will not cool, or it will run but the compressor or fan will not start. You may hear a humming sound from the outdoor unit without the system actually running.

Capacitors typically last 10 to 20 years, but heat, power surges, and age wear them out faster. If your AC stops working and an HVAC technician tells you the capacitor has failed, you can replace it yourself if you are comfortable working with electrical components. The part itself costs $15 to $50, but a service call to have a technician replace it runs $150 to $300.

This is one of the few AC repairs a homeowner can safely do without a license, because you are not touching refrigerant lines or the compressor itself. However, capacitors hold an electrical charge even when the unit is off, so you must discharge it before touching it.

Key Takeaways

  • Turn off power to the AC unit at the breaker and wait at least five minutes before opening the outdoor unit to let the capacitor discharge.
  • Discharge the capacitor manually using an insulated screwdriver across the terminals, or use a capacitor discharge tool if you have one.
  • Take a photo of the wire connections before removing the old capacitor so you can reconnect the new one in the same order.
  • Buy a replacement capacitor with the exact same microfarad (µF) rating printed on the old part, or bring the old capacitor to the supply store to match it.
  • If the capacitor is swollen, leaking, or burned, replace it immediately; if it simply tests low, you can wait for the next scheduled maintenance.

Gather the right tools and the correct replacement part

You will need an insulated screwdriver (one with a plastic or rubber handle), a multimeter set to capacitance mode (optional but helpful for testing), and the replacement capacitor. Some people use a capacitor discharge tool, which is a specialized device that safely drains the charge, but an insulated screwdriver works if you are careful.

The most important step is buying the correct capacitor. Look at the label on your old capacitor and write down the microfarad (µF) rating—it will say something like "45 µF" or "50 µF." The voltage rating (usually 370V or 440V) must also match. If you cannot read the label or the capacitor is burned, bring the old part to an HVAC supply store or a big-box hardware store and ask them to match it. Do not guess; installing the wrong capacitor can damage your compressor.

Some AC units have two capacitors: one for the compressor and one for the fan motor. If both have failed, you will need to replace both. The process is the same for each.

Turn off power and discharge the capacitor safely

Turn off the AC unit at the thermostat, then switch off the breaker that controls the outdoor unit at your electrical panel. Wait at least five minutes. This gives any residual charge time to bleed off, but the capacitor will still hold a charge, so you must discharge it manually before touching the terminals.

Use an insulated screwdriver to touch both terminals of the capacitor at the same time. You may see a small spark—this is normal and means the capacitor is discharging. Do this two or three times to be sure. If you have a capacitor discharge tool, follow its instructions instead. Never skip this step; a shock from a capacitor can be painful or dangerous.

Once the capacitor is discharged, take a clear photo of the wire connections before you disconnect anything. This photo is your map for putting the new capacitor in the right way. Label the wires in your photo if the connections are not obvious.

Remove the old capacitor and install the new one

Disconnect the wires from the old capacitor by loosening the terminal screws with your screwdriver. Keep track of which wire came from where—this is why the photo matters. Some wires may be labeled with small tags; if so, note what they say.

Once all wires are free, unscrew or unclip the capacitor from its mounting bracket and remove it. Set it aside safely. Do not throw it in the trash; many hardware stores and HVAC suppliers will take old capacitors for recycling.

Position the new capacitor in the same location and secure it to the bracket with the same screw or clip. Then reconnect the wires to the terminals in the exact order shown in your photo. Tighten each terminal screw firmly so the wire does not slip off. Double-check that each wire is seated properly before you close the unit.

Test the system and know when to call a technician

Close the outdoor unit and restore power at the breaker. Turn the AC back on at the thermostat and listen for the compressor and fan to start within a few seconds. The unit should begin cooling. If it does not start, turn it off immediately and check that all wire connections are tight and in the right order.

If the system still does not run after you have checked the connections, the problem may not be the capacitor, or there may be a second failed part. At that point, call an HVAC technician. Do not keep trying to restart the unit, because running it without a working capacitor can damage the compressor.

If the new capacitor works but the system cycles on and off frequently, or if the compressor hums but does not start, there may be a deeper electrical issue. These are signs to have a professional inspect the unit.

When to replace a capacitor versus when to wait

A capacitor that is visibly swollen, leaking fluid, or burned should be replaced immediately. These are signs of imminent failure. A capacitor that tests low on a multimeter but shows no physical damage can often wait until your next scheduled maintenance visit, especially if the AC is still cooling.

However, if your AC is not cooling at all and a technician has confirmed the capacitor is the problem, replace it as soon as you can. Running the compressor without a working capacitor will shorten its life and lead to a much more expensive repair.

If you are not confident working with electrical components or if you are uncomfortable opening the outdoor unit, hiring a technician is the safer choice. The cost difference is not large enough to justify a mistake that could damage the compressor or injure you.

Frequently Asked Questions

Can I replace a capacitor while the AC is running?

No. Always turn off the AC at the thermostat and switch off the breaker before you open the unit. Even with the breaker off, the capacitor holds a charge, which is why you must discharge it manually. Working on a live electrical system is dangerous.

What if I connect the wires in the wrong order?

The system will not start, or it may start and then shut down quickly. Turn off the power immediately and check your photo to correct the connections. Wires connected backward can damage the compressor or fan motor over time, so get it right before running the unit again.

How do I know if the capacitor is the problem and not something else?

If the compressor hums but does not start, or if the fan does not spin, the capacitor is usually the culprit. If nothing happens at all—no hum, no sound—the problem may be a breaker, a contactor, or a wiring issue. A multimeter can test the capacitor, but if you are unsure, a technician can diagnose it in minutes.

Do I need a license to replace an AC capacitor?

No. Replacing a capacitor does not require an HVAC license in most states because you are not working with refrigerant or the sealed system. However, you are working with electrical components, so follow safety steps carefully and do not proceed if you are uncomfortable with the task.

Can a bad capacitor damage my compressor?

Yes. If the capacitor fails and you keep running the AC, the compressor will work harder to start and may overheat. This shortens its life and can lead to a compressor replacement, which costs $800 to $2,500. Replace a failed capacitor quickly to avoid this.