Aircon Capacitor: What It Does and the Signs It Has Failed

If your outdoor unit is humming and its fan is standing still, read the signs below first. If they match, it is a parts job: our aircon parts replacement service tests the capacitor at your house and fits the right one.

If the whole unit is dead rather than humming, start with aircon not turning on, and if it began after a brownout, the AVR for aircon guide covers what low voltage does.

What the capacitor does

It stores electrical energy and lets it go in one push to start the motors, then keeps feeding them a steady supply so they run smoothly. The compressor and the outdoor fan both depend on it.

There are two kinds. A start capacitor gives a short, strong push to get a motor turning. A run capacitor stays in the circuit the whole time the motor is running. Many units carry a dual run capacitor: two capacitors in one casing, one for the compressor and one for the fan, which is why one failed part can stop both.

Signs the capacitor has failed

The outdoor unit hums but the fan does not turn.

The motor is getting power and cannot start. Carrier names a failed capacitor as the usual cause, with a failing motor as the other.

The aircon is slow to start a cycle, or will not start at all.

The compressor needs the capacitor’s push to get going. Without it, it tries and gives up.

It runs but the air is not cold.

The indoor fan blows, and the outdoor side is not doing its share because the compressor or the outdoor fan is not running properly.

It switches itself off, or cycles on and off.

A motor that struggles to start or to keep running trips out and tries again.

A burning smell or smoke at the outdoor unit.

Switch it off at the breaker and leave it off until it has been looked at.

None of these proves it on its own. A failing fan motor, a contactor or a fault on the board can look the same from the outside, which is why the capacitor gets a meter reading before anything is replaced.

• 1Flat top, dry casing. How a working run capacitor looks. Looking fine does not prove it is fine, because one can lose capacity without changing shape, so a meter reading against its rated microfarads is the real test.
• 2Domed top, cracks or liquid oozing out. A capacitor that looks like this has failed or is about to. It is replaced with one of the rating printed on the old one's label.

Why capacitors fail

Heat and vibration, mostly. Carrier puts the usual life of an aircon capacitor at 10 to 20 years, depending on how hard the unit works and how much heat it sits in. An outdoor unit in full afternoon sun, running most of the day, is the hard end of that range.

When the fan will not spin

A fan that hums and does not turn is getting power and cannot start, and the capacitor that gives it its starting push is the first suspect. If the capacitor reads correctly and the fan motor is silent, hot to the touch or smells of burning, the motor itself is the likelier fault. Both are on the list of parts we replace, and the test decides which one you pay for.

• 1Out of range. The reading is more than 10 percent off the rating on the label, so the capacitor is replaced and the fan is tried again.
• 2In range. The capacitor is fine. A motor that is silent, hot to the touch or smells of burning is the fault instead.

Why this is not a DIY job

A capacitor holds a high-voltage charge after the aircon is switched off, and Trane and Carrier both warn that touching one can cause serious injury or death. Videos make the swap look like four wires and a bracket. The step they rush is the one that matters: discharging it safely before anyone touches the terminals.

How a technician checks and replaces it

1. Power off at the breaker, and the service panel opened.
2. The capacitor discharged before it is handled.
3. A look for bulging, cracks or leaking.
4. A meter reading against the rating on its label. TCL’s guide gives the rule: the reading should be within 10 percent of the rated value.
5. If it is out, a replacement with the same rating, wired to the same terminals.
6. The aircon run, and the compressor and fan watched as they start.

Questions people ask

How do you know if your AC capacitor is bad?

The common signs are an outdoor unit that hums while its fan does not turn, an aircon that is slow to start or will not start, air that is not cold, and a unit that keeps switching itself off. A capacitor that has bulged at the top, cracked or leaked has visibly failed. A technician confirms it with a meter, because one can lose capacity without looking any different.

What is the purpose of a capacitor in an aircon?

It stores electrical energy and releases it to start the compressor and fan motors, then keeps feeding them a steady supply so they run smoothly. Many units have a dual run capacitor, which is two capacitors in one casing, one for the compressor and one for the outdoor fan.

Can I replace my AC capacitor myself?

No. Carrier and Trane both advise homeowners not to, because a capacitor holds a high-voltage charge that can cause serious injury or death, even after the aircon has been switched off. It has to be discharged safely before anyone touches it, and that is a technician’s job.

Can an aircon run with a bad capacitor?

Sometimes, and badly. A weak capacitor can let the unit start and run without cooling properly, and Carrier lists higher electricity use and frequent cycling among the signs. A motor that keeps trying to start without its push is under strain, so switch the aircon off and have it checked rather than keep running it.

Is the fan motor broken if the fan will not spin?

Not necessarily. A fan that hums and does not turn is often the capacitor, which is why it is tested first. If the capacitor reads correctly and the motor is silent, hot or smells of burning, the motor is the likelier fault.

Have it tested at home

Tell us what the outdoor unit is doing and the town you are in. A technician comes to the house, tests the capacitor and the fan motor, and gives you a quote before any part goes in. See what the aircon parts replacement service covers.

Other guides

• Aircon freezing up: why ice forms and what to do

  Ice on the indoor coil or the pipes, how to thaw it safely, and the three causes behind it.

• Aircon making noise: which sounds are normal and which are not

  Clicks and water sounds that are normal, and the rattles, hisses and squeals that need a technician.

• Aircon not turning on: what to check, and the 3-minute wait after a brownout

  Remote, breaker, the compressor’s restart delay, and the flashing lamp that means a technician.

• AVR for aircon: when you need one and how to size it

  What an AVR does in a brownout, which units already protect themselves, and sizing it from the nameplate.

• Aircon watts: how many watts a 1 HP aircon uses, size by size

  Running watts and kWh per hour for each size, how to read the nameplate, and starting watts for a generator.

• Types of aircon: split type, window type, portable and centralized

  How each is installed, how noisy and costly to run it is, which rooms suit it, and what a clean has to reach.

• What is freon, and why your aircon should not need a yearly refill

  The sealed loop, why low gas means a leak, the three refrigerants in Philippine units, and the refill myth.

• Aircon HP and room size: what each size cools, 0.5 to 2.5 HP

  The square metres each size suits in a shaded room and a hot one, with the arithmetic behind 0.6 HP.

• Aircon cleaning schedule: what to clean, and how often

  The filter every two weeks, a full clean by how hard the unit works, the deeper clean, and when in the year to book it.

• Common aircon problems, symptom by symptom

  Not cold, weak air, water inside, a smell, ice, noise, tripping and blinking: what each points to and what you can check.

• Aircon lifespan: how long it lasts, and repair or replace

  How many years a unit usually gives, what shortens it, and the signs that a repair is no longer worth it.

• Inverter vs non-inverter aircon: which costs less to run

  Kilowatt hours side by side at 2, 4, 8 and 12 hours a day, and the kind of use where a non-inverter is still fine.

• What size aircon for a bedroom, living room or kitchen

  Room by room: what each kind of room adds to the size you need, worked through for a sample two-storey house.

• Aircon energy saving tips that lower the bill

  Where the heat gets in, the setting and the fan, the timer, the cleaning, and reading the DOE energy label.

Sources: Carrier, “What is an AC Capacitor?” and “AC Fan Not Spinning?”; Trane, “AC Capacitors: What They Are and Why They Matter” and its heat pump capacitor guide; TCL, “How to Diagnose and Replace Your AC Capacitor”.