AC Capacitor: How to Tell, and Which One to Buy

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

AC Capacitor: How to Tell, and Which One to Buy

Short answer

A hum with the fan sitting still, a fan that needs a push to start, or a bulged top are the three signs. The replacement has to match the microfarad numbers exactly and can have the same or a higher voltage, never lower. It can also hold a charge after the power is off, and that part is not optional.

An AC capacitor is a small metal tin inside the outdoor unit that gives the fan and the compressor the shove they need to start turning. It is one of the cheapest parts on the system and a common one to fail.

It is also the part that can hurt you, because it can store a charge after the power is off. Everything on this page assumes you deal with that first.

Before you start

What to carry: a screwdriver, your phone, a long stick and a 15,000 ohm, 2 watt resistor if you are going to do the work yourself. Twenty minutes.

Gas or electric? It makes no difference to this part. The capacitor is in the outdoor unit, which is the same box whether the heating indoors is a gas furnace or an electric air handler, and the same box on a heat pump.

Know where you will stop. Identifying a failed capacitor and buying the right one is a reasonable thing to do at home. Fitting it means working inside a box that carries mains voltage, and plenty of people read this far, order the part and have somebody fit it. That is a perfectly sensible place to draw the line.

The one thing that makes this different from a filter. A capacitor stores electricity the way a small battery does, and Carrier warns it can hold that charge when the power is off. Touching the terminals, or letting a screwdriver bridge them by accident, gives you a serious belt.

So the power goes off in two places, the breaker and the pull out block on the wall, and then the capacitor gets discharged deliberately before a hand goes near it.

There is also mains voltage on the contactor a few inches away, which does not care that the capacitor is discharged.

Is it actually the capacitor?

Check 1Free, five minutes

Three signs, and two of them you can see from outside

It hums but the fan does not turn. Power is reaching the motor and there is not enough push to get it moving.

The fan starts if you give it a nudge. With the power off, reach through the top grille with a long stick and spin the blade, then restore power. A fan that runs after a push and not before it points to the capacitor.

The top of the tin is bulged or leaking. A healthy capacitor is flat on top. One domed upwards, split, or with an oily residue round it has failed, and that one needs no test at all.

The same checks in order for a unit that will not start at all are on the outdoor unit page, because a dead outdoor unit is not always this part.

A fan that has to be pushed is not a fix. Leaving a unit running when the motor cannot start on its own draws heavy current through a motor that is not turning, and it cooks the windings. A capacitor is a far cheaper part than a fan motor.

Switch it off at the breaker until the part arrives.

Reading the old one so you buy the right one

Everything you need is printed on the side. The numbers have to match, the volts may go up.

Check 2

Copy three things off the tin

The microfarad numbers. Most outdoor units use a dual capacitor doing two jobs at once, and it is printed as something like 45 + 5 MFD. The big number runs the compressor, the small one runs the fan. Both have to match exactly. There is a small tolerance built in and you do not use it deliberately.

The voltage. Usually 370 or 440 VAC. You may fit a higher one than the original, so a 440 in place of a 370 is fine and often what is on the shelf. You may never fit a lower one.

The terminals. HERM for the compressor, FAN for the fan, C for common. Photograph the wires before you touch anything, because there are usually more wires than terminals and several look alike.

Shape does not matter electrically. Round or oval, whichever fits the strap that holds it.

Changing it, if you are doing that part

Two ways of cutting the power, then a deliberate discharge, and only then a hand inside the box.

Check 3

Power off twice, then discharge it

Switch the breaker off and pull the disconnect block out of its box on the wall. Take the block with you rather than leaving it balanced on top, so nobody puts it back while you are working.

Take the service panel off the unit. The capacitor is the tin with the wires on top.

Then discharge it. A 15,000 ohm, 2 watt resistor, the size Carrier’s service manual gives, with a lead on each end, held across two terminals at a time for about five seconds, does it safely and slowly. Do HERM to C, then FAN to C, then HERM to FAN.

Shorting a capacitor with a screwdriver is the version everybody has seen online. It works, it also throws a spark, can weld the blade to the terminal and is how people get hurt. A resistor is cheap, and it does not throw that spark.

Check 4

Photograph, swap, strap, close up

Photograph the terminals with every wire still on. Then pull the spade connectors off one at a time, undo the metal strap, and lift the old one out.

The new one goes in the same way round, the strap goes back, and every wire returns to the terminal it came off. Push the spades fully home so they grip.

Panel back on, disconnect block back in, breaker on, and watch a full start. The fan should come up to speed on its own within a second or two.

When it keeps failing

A capacitor that dies every summer is not really the fault. It is a symptom.

Heat. They are rated by temperature as much as by voltage, and a unit baking in full sun against a wall gets through them. Shade and clearance help.

A compressor that is getting hard to start. An ageing compressor pulls harder and harder on start up, and it takes the capacitor with it. That is the one that ends in a new system.

A cheap part. There is a real difference between capacitors, and the very cheapest do not last a season. It is worth buying a known make for a part that costs this little.

Low voltage in the house, or a long run of undersized cable, makes every start harder on everything.

Questions people ask

Can I use a 45 + 5 in place of a 40 + 5?

No. The microfarad numbers are not a maximum, they are what the motor was designed around, and the wrong size runs it hot. Match them.

Mine is a single capacitor, not a dual.

Some systems use two separate ones, one for the fan and one for the compressor. Same rules, and each has its own rating printed on it.

How long should a capacitor last?

Carrier says most last 10 to 20 years. Bryant’s own guide gives 5 to 10 years in one place and 10 to 20 in another, and both makers say heat shortens it. They are considered a wear part rather than a failure.

The fan runs but the compressor does not.

On a dual capacitor the compressor side can fail while the fan side is fine. The contactor and the compressor’s own overload do the same thing, so it is worth reading through the whole not cooling list.

Is it worth keeping a spare?

On an older system, yes. They fail in heatwaves, which is exactly when nobody can get to you.

How do I test one with a meter?

You need a meter with a capacitance range, and you test it disconnected and discharged. Carrier’s service manual says to replace one that reads more than 10 percent away from the printed number. Without that range on the meter, the push test and the bulged top are what you have.

Sources: Carrier service manual 24-25-9SM (capacitor checks, bulging or leaking cases, 15,000 ohm 2 watt discharge resistor, 10 percent tolerance), Goodman service instructions RS6200006 (COM, HERM and FAN terminals, discharge before handling), Specap "370V vs 440V Capacitors" (same microfarads, voltage may go up), Bryant "What to Do When Your Bryant AC Fan Is Not Spinning" (hum with the blades still, bulge or leak), Carrier "AC Unit Not Turning On" and "What is an AC Capacitor?" (10 to 20 year life) and Bryant "AC Capacitor Replacement Guide" (5 to 10 years in its questions, 10 to 20 in its text)