What a multimeter tells you about a capacitor

A capacitor is a component that stores electrical charge and releases it when your air conditioner needs a power boost to start the compressor or fan motor. When a capacitor fails, your AC won't start or will run weakly. A multimeter — a handheld meter that measures voltage, resistance, and other electrical properties — can tell you whether a capacitor is holding a charge, which is the first sign it is working.

The test does not tell you everything. A capacitor can hold a charge and still be failing slowly, or it can fail only when hot. But if a capacitor shows no charge at all, it is definitely dead. If it shows a charge building up and holding, it is probably good enough to keep using for now.

This test is safe to do at home if you follow the steps exactly. The danger is that a capacitor can hold a dangerous electrical charge even when the unit is off. You will discharge it before testing, which removes that risk.

Key Takeaways

  • Always turn off the AC unit and wait five minutes before touching any capacitor, then discharge it by touching a screwdriver across both terminals to remove stored charge.
  • Set your multimeter to the capacitance setting (marked with the symbol that looks like two parallel lines) before you begin testing.
  • Touch the red probe to the positive terminal and the black probe to the negative terminal, then read the number on the display — it should match or come close to the microfarad (µF) number printed on the capacitor's label.
  • If the reading is zero or far below the label number, the capacitor is dead and needs replacement; if it is within 10 percent of the label number, the capacitor is holding charge normally.
  • Do not test a capacitor while the AC is running, and do not touch both terminals at the same time with your bare hands.

Turn off power and discharge the capacitor first

Before you touch anything, switch off your AC unit at the thermostat and at the breaker box. Wait at least five minutes. This gives any remaining charge time to bleed away naturally, though capacitors can hold charge for hours.

Next, discharge the capacitor by hand. Take an insulated screwdriver — one with a plastic handle — and touch the metal tip across both terminals of the capacitor at the same time. You may see a small spark or hear a pop. That is the stored charge leaving safely. Do this even if you think the capacitor is dead, because you cannot tell by looking.

After discharging, do not touch the terminals with your bare hands. Wear work gloves or keep your hands away from the metal parts. The capacitor should now be safe to test.

Set your multimeter to capacitance mode

Look at the dial or screen on your multimeter. You are looking for a symbol that looks like two parallel horizontal lines — this is the capacitance setting. On analog multimeters, it is usually marked with the Greek letter µ (mu) or the word "Farad" or "F". On digital multimeters, it is often a button labeled "CAP" or a dial position labeled with the parallel-lines symbol.

Turn the dial to the capacitance setting. If your multimeter has a range selector (a dial with different numbers like 2000, 20000, 200000), start with the highest range. You can move to a lower range if the reading is too small to read clearly. Many modern multimeters auto-range, meaning they pick the right setting for you — if yours does, you do not need to adjust anything after selecting capacitance mode.

Connect the probes and read the measurement

Look at the capacitor terminals. One is marked with a plus sign (+) or a stripe, and one is unmarked or has a minus sign (−). The plus terminal is positive, and the other is negative. This matters — you must connect the probes the right way around.

Touch the red probe to the positive terminal and the black probe to the negative terminal. Hold them steady for two to three seconds. The multimeter display will show a number. This number is the capacitance in microfarads, written as µF. Write down what you see.

Now look at the label printed on the capacitor itself. It will say something like "45 µF" or "5 µF + 5 µF" (if it is a dual capacitor). Compare your reading to this number. If your reading is within 10 percent of the label number — for example, if the label says 45 µF and you read between 40 and 50 µF — the capacitor is holding charge normally and is probably still working.

What the results mean

If your multimeter shows a reading that matches the label number or comes close (within 10 percent), the capacitor is functioning. It is storing and holding charge the way it should. You can put the unit back together and turn the AC back on.

If your multimeter shows zero, or a number far below the label (more than 10 percent lower), the capacitor is dead or dying. A reading of zero means it is not holding any charge at all. A reading that is 20, 30, or 50 percent below the label number means it is losing its ability to store charge and will fail soon. In either case, the capacitor should be replaced.

If your multimeter shows a number higher than the label, this is unusual but not always a problem — it can happen with older capacitors or if the meter is reading residual charge. Discharge the capacitor again and test a second time. If the second reading is still high, the capacitor may be failing in a different way and should be replaced.

Why this test has limits

This test tells you whether a capacitor is holding a static charge right now, but it does not tell you how the capacitor behaves when the AC is running and hot. A capacitor can pass this test and still fail when the compressor tries to start, especially if it is old or has been exposed to heat stress. This test is a first check, not a final diagnosis.

If your AC is not starting or is running weakly, and this test shows the capacitor is good, the problem may be elsewhere — a bad contactor, a tripped breaker, a refrigerant leak, or a motor bearing that is stuck. A professional technician can test the capacitor under load (while the AC is trying to run) and check other components at the same time.

Frequently Asked Questions

Can I test a capacitor while the AC is running?

No. Testing a live capacitor is dangerous and will damage your multimeter. Always turn off the unit and wait five minutes before testing. If you need to know whether the capacitor is failing while the AC runs, a technician with specialized equipment can do that safely.

What if the capacitor has three terminals instead of two?

You have a dual capacitor, which has two capacitors in one case. It will have a label like "5 µF + 5 µF" or "45 + 5 µF". Test each capacitor separately: red probe to the positive terminal, black probe to the first negative terminal, then test again with the black probe on the second negative terminal. Compare each reading to its matching number on the label.

Do I need to replace the capacitor myself or call a technician?

If the test shows the capacitor is dead, you can replace it yourself if you are comfortable working with electrical components and follow safety steps carefully. Capacitors are inexpensive (usually $15 to $50) and the replacement takes 10 minutes. If you are unsure, a technician can do it quickly and will also check whether anything else caused the capacitor to fail.

Why did my capacitor fail?

Capacitors fail from age, heat, power surges, or a motor that is drawing too much current. If your AC is more than 10 years old, capacitor failure is normal wear. If a new capacitor fails within a year, something else in the unit may be overloading it — a technician should check the motor and compressor.

Can a bad capacitor damage my AC compressor?

Yes. If a capacitor fails completely, the compressor cannot start and the motor may overheat trying. If the capacitor is failing slowly, the compressor runs weakly and may not cool your home. Replacing a failed capacitor quickly prevents damage to more expensive parts.