How to Check and Replace an AC Capacitor

A weak or bulging capacitor is behind most outdoor units that hum but won’t start — swapping it out to replace AC capacitor failures like this is usually a 20-minute fix running $15-40 in parts versus $150-300 for a service call. Match the microfarad rating and voltage exactly, discharge the old one safely, and move the wires over one terminal at a time.

What a Capacitor Actually Does — And Why It Fails

A capacitor is a small energy reservoir mounted next to your outdoor unit’s compressor. It stores a charge and releases it in one quick burst that gets the compressor and fan motor spinning, then feeds a steady secondary current while they run. Heat wears it out — the unit cycles on and off many times on a hot afternoon, and each cycle stresses the foil inside the can. After a few years that foil breaks down, capacitance drops below spec, and the compressor either hums without starting or doesn’t try at all. Most people hear that hum and assume the compressor is dying — it isn’t, it’s just the part meant to kick that compressor into motion, now degraded and leaving the motor humming under a locked rotor with nowhere to go. I’ve run into that same hum-and-stall pattern plenty of times over 30+ years of hands-on repair work, and the physics doesn’t change just because the box looks different.

If your unit is short cycling instead of humming, that’s usually a different problem. A marginal capacitor can still cause both symptoms depending on how far it’s degraded.

How to Check and Replace an AC Capacitor — step-by-step repair infographic

Capacitor Swap at a Glance

• 1. Kill power — breaker + outdoor disconnect, confirmed with a voltage tester
• 2. Discharge — 20,000-ohm resistor across terminals, 5 seconds
• 3. Photograph wiring — HERM, FAN, and C terminals before anything moves
• 4. Pull the old part — release the mounting strap, remove the can
• 5. Match the label — same µF and voltage, no rounding up
• 6. Reconnect and test — one wire at a time, then restore power

Total time: ~20 minutes once the panel is off  |  Part cost: $15-40  |  Service call for the same fix: $150-300

Is It Really the Capacitor? Check These Signs First

Most capacitor failures are just weak — the can looks fine, but a multimeter reading comes in noticeably under the printed microfarad rating. Many others are visibly bulging or leaking a brown residue out the top or bottom seam. That one’s obvious the second you pop the access panel off. A smaller share come down to a wrong replacement bought in the first place, or a mismatch discovered after the swap — more on that below. That hum-and-stall pattern tracks back to a tired capacitor more often than not.

You’ll usually hear it before you see it. Listen for a steady electrical hum from the outdoor unit with no fan movement and no compressor kick. Sometimes an overload relay clicks and gives up after a few seconds. Feel the can itself once you’ve killed power. If the top or bottom is rounded outward instead of flat, that’s your answer.

The wrong fix I see people try more than any other is touching a screwdriver blade across both terminals to “discharge” the capacitor, sometimes with the power still connected somewhere in the circuit. That arc doesn’t just risk a shock. It pits the terminals and can throw off your multimeter reading on the very capacitor you’re trying to test. Use a 20,000-ohm, 2-watt resistor with insulated leads across the terminals for about 5 seconds instead. It’s a two-dollar part and it does the job without cooking anything. I’ve got a full walkthrough on how to safely discharge a capacitor if you want the step-by-step on that alone.

That video covers the same discharge-and-test sequence I just described, plus shows what a good capacitance reading looks like compared to a bad one. Watch it once before you crack the panel open if this is your first time. It saves a lot of guessing.

How Do You Test a Capacitor With a Multimeter?

Testing a capacitor is a short, simple sequence. Follow it in order and don’t skip the label comparison:

1. Set the multimeter to capacitance mode (the µF symbol).
2. Touch the leads to the capacitor’s two (or three) terminals.
3. Read the number and compare it against what’s printed on the capacitor’s own label — not a number from memory, not a guess.
4. Check the tolerance range printed next to the rating. A reading outside that range means the part is done, full stop.

This is the step I used to skip early on. I’d eyeball the can for bulging and call it good if it looked fine. Then I got burned a couple of times by a capacitor that looked perfectly normal but had already dropped well under its rated capacitance. I test every one now, bulging or not.

Replacing a Single Capacitor — Step by Step

1. Confirm power is off at the breaker and the outdoor disconnect. Test with a non-contact voltage tester before you touch anything.
2. Remove the access panel — usually two or four Phillips screws on the side of the condenser cabinet.
3. Discharge the old capacitor with an insulated resistor across the terminals for 5 seconds (see the discharge guide linked above).
4. Photograph the wiring before you disconnect anything — trust me on this one, you will not remember which wire went to HERM versus FAN twenty minutes from now.
5. Remove the mounting strap and pull the old capacitor free.
6. Match the new capacitor to the exact µF rating(s) and voltage printed on the old one — never round up “close enough.”
7. Reconnect the wires one terminal at a time using your photo as reference, and secure the new mounting strap.
8. Restore power and confirm the compressor and fan both start cleanly on the next thermostat call.

Budget about 20 minutes once you’ve got the panel off and the old part in hand. The first time, give yourself closer to 45 minutes for the extra care on wire ID.

The Part Almost Every Guide Skips: Dual-Run Terminal Layout

If your unit uses a dual-run capacitor — one can handling both the compressor (HERM) and the fan (FAN) off a shared common (C) terminal — the replacement part doesn’t always match the physical terminal arrangement of the one you pulled. This can happen even when the µF and voltage numbers match exactly. Some manufacturers rotate the tab layout 90 degrees or swap which side FAN and HERM sit on. Your photo from step 4 tells you which wire goes where by function, not by physical position. Don’t just plug wires back into “the terminal that looks like where it came from” if the new can is laid out differently.

Here’s a trick that helps when the exact dual-run combo isn’t sitting on the shelf at your local supply house: you can usually replace one dual-run capacitor with two separate single-run capacitors. Wire one at the compressor’s rated µF and one at the fan’s rated µF, both tied to the same common point. It takes a few extra minutes of wiring and you’ll need a slightly bigger mounting bracket or a zip-tie solution, but it gets you running that same day instead of waiting on a special order. This workaround comes up often on older Trane and Carrier units where the factory dual-run value just isn’t a common stock item anymore.

Matching the Right Part to Your Unit

Carrier and Bryant condensers commonly run dual-run capacitors in the 35/5 to 45/5 µF range at 370-440V. RepairClinic’s Carrier capacitor listings are a good place to cross-reference by model number. Lennox units frequently spec a run capacitor like the 89M80 run capacitor, which runs around $25-35 depending on the supplier. Whatever the brand, the label on the old part is the only spec sheet that matters. Model-number lookups get you in the right neighborhood, but the printed µF and voltage on the can you’re removing is the number you buy against.

When to Call a Pro Instead

Keeping the New Capacitor From Failing Early

Capacitors don’t fail randomly. Heat and voltage spikes age them, and both are things you can partly control. Keep the condenser coils clean. Keep the area around the unit clear too, so it isn’t working harder than it needs to just to move air. This matters more in Phoenix-area heat than almost anywhere else. If your unit is not blowing cold air even after a good capacitor swap, check the coils or refrigerant charge next. A yearly capacitance check during spring maintenance takes five minutes with a meter. It catches a weak capacitor months before it strands you on a 110-degree afternoon.

Can I replace an AC capacitor myself without HVAC training?

Yes, in most cases. The two things that actually matter are discharging it safely and matching the µF/voltage exactly. If you can follow those two rules, the rest is just screws and wire terminals.

How do I know if it’s a start capacitor or a run capacitor?

A run capacitor stays in the circuit the whole time the motor runs. It’s usually the oval or round can you see mounted near the compressor. A start capacitor works for a shorter burst, is more common on hard-starting compressors or older single-phase motors, and is often paired with a start relay. Check the can’s label — it’ll say which one it is.

What happens if I install the wrong microfarad rating?

Too low and the motor won’t get enough starting torque. You’ll hear the same hum-and-no-start symptom you started with. Too high and you risk overheating the motor windings over time. Match the number on the label. Don’t round.

Is it safe to touch a capacitor after the unit has been unplugged for hours?

No. It doesn’t matter how long it’s been sitting — treat every capacitor as charged until you’ve personally discharged it with a resistor. Readings on “dead” capacitors that had been unpowered for over a week still come back showing a stored charge often enough that it’s not worth the risk.

Can a bad capacitor damage my compressor if I don’t replace it?

Eventually, yes. A compressor repeatedly trying to start against a weak or dead capacitor draws way more amperage than it’s designed for, and that heat buildup shortens the motor’s life. A $30 capacitor fix left too long can turn into a compressor replacement costing well over a thousand dollars.