What a capacitor does and why it fails

A heat pump capacitor stores electrical energy and releases it to start the compressor and fan motors. When a capacitor fails, your heat pump won't start at all, or it will hum without running. The capacitor is a cylindrical component about the size of a soda can, usually mounted on the outdoor unit near the compressor.

Capacitors fail because they wear out over time—typically after 10 to 20 years—or because of power surges, extreme heat, or a short in the motor itself. A failed capacitor is one of the most common heat pump repairs, and testing it yourself can tell you whether it's the problem before you call a technician.

Testing requires a digital multimeter (a tool that measures voltage and resistance) and takes about five minutes. The test is safe if you follow the discharge step, which removes any stored charge before you touch the capacitor terminals.

Key Takeaways

  • A failed capacitor will cause your heat pump to hum but not start, or not start at all.
  • You will need a digital multimeter set to the capacitance setting (marked with the symbol that looks like two parallel lines).
  • Always discharge the capacitor by touching a screwdriver across both terminals before testing, even if the unit is off.
  • A reading between 5 percent below and 5 percent above the number printed on the capacitor means it is working; anything outside that range means it has failed.
  • If the capacitor has failed, you should call a technician to replace it rather than attempting it yourself, because the compressor may also be damaged.

Gather your tools and turn off power

You will need a digital multimeter with a capacitance setting. Most basic multimeters cost $15 to $30 and have this feature. The capacitance setting is marked with a symbol that looks like two parallel lines (the capacitor symbol) or the letter F with a line underneath it.

Turn off the breaker that controls the outdoor unit at your electrical panel. Wait five minutes to allow any charge to drain. Do not rely on the thermostat alone—you must cut power at the breaker.

Open the access panel on the outdoor unit. The capacitor is mounted inside near the compressor. Take a photo of the wire connections before you disconnect anything, so you know how to reconnect them later.

Discharge the capacitor safely

Even though the power is off, a capacitor can hold an electrical charge and deliver a shock. Discharging it takes 10 seconds and is the most important safety step.

Take an insulated screwdriver (one with a plastic handle) and touch the metal shaft across both terminals of the capacitor at the same time. You may see a small spark—this is normal and means the capacitor had a charge. Hold the screwdriver in place for a few seconds to may support the charge is fully released.

If you do not see a spark, the capacitor may already be dead, but discharge it anyway as a precaution.

Disconnect the capacitor and set up the multimeter

Once the capacitor is discharged, use a screwdriver or needle-nose pliers to carefully pull the wire terminals off the capacitor posts. Keep track of which wire came from which post—the photo you took earlier will help.

Set your multimeter to the capacitance setting. This is usually marked with the capacitor symbol or the letter F. If your multimeter has a range selector, choose the range that matches or exceeds the capacitor's rating. For example, if the capacitor is rated 45 microfarads (µF), set the multimeter to 50 µF or higher.

Some multimeters auto-range, meaning they adjust the range automatically. If yours does, you can skip the range selection step.

Test the capacitor and read the result

Touch one probe of the multimeter to each terminal of the capacitor. It does not matter which probe touches which terminal—capacitors work the same way in both directions.

Read the number on the display. The capacitor's rating is printed on its body in microfarads (µF). A healthy capacitor will read within 5 percent above or below that number. For example, a 45 µF capacitor should read between 42.75 µF and 47.25 µF.

If the reading is much lower than the rating, or if the multimeter shows 0 or OL (overload), the capacitor has failed and needs to be replaced.

What the test results mean

A reading within 5 percent of the rated value means the capacitor is functioning. If your heat pump is still not running, the problem is elsewhere—possibly a contactor, a compressor, or a wiring issue. In that case, you will need a technician to diagnose further.

A reading significantly below the rating, or a reading of 0, means the capacitor has lost its ability to store charge and must be replaced. A reading much higher than the rating is rare but also indicates failure.

If the multimeter shows OL (overload) or refuses to settle on a number, the capacitor is likely shorted internally and has failed.

When to call a professional

If your test shows the capacitor has failed, do not attempt to replace it yourself. Capacitor replacement requires opening the sealed refrigerant circuit, and if the compressor is damaged, replacing only the capacitor will not fix the problem. A technician can test the compressor at the same time and advise whether replacement is worth the cost.

If the capacitor tests good but your heat pump still will not start, the problem is a contactor, compressor, or control board. These require professional diagnosis and repair.

If you are uncomfortable working inside the outdoor unit or handling the multimeter, a technician can run this test for you in a few minutes. The cost is usually $75 to $150, which is reasonable insurance against damaging the unit or yourself.

Frequently Asked Questions

Can I test the capacitor while the power is on?

No. Always turn off the breaker first. Testing a live capacitor can damage your multimeter and create a shock hazard. Even after the power is off, discharge the capacitor with a screwdriver before touching it.

What if the capacitor reads slightly low but not below 5 percent?

A reading within 5 percent of the rated value is normal and acceptable. Capacitors naturally drift slightly over time. If the reading is within range, the capacitor is working and is not the cause of your problem.

Do I need to replace the capacitor myself, or should I wait for a technician?

You should wait for a technician. Capacitor replacement involves opening the refrigerant circuit, which requires special tools and certification. A technician can also test the compressor to make sure it is not damaged, which affects whether replacement is worth doing.

How much does a capacitor replacement cost?

Capacitor replacement typically costs $200 to $400 including labor and the part. Costs vary by region and whether the technician finds other problems during the service call.

Can a bad capacitor damage the compressor?

A failed capacitor can cause the compressor to work harder or fail to start, which may damage it over time. This is why it is important to replace a failed capacitor promptly and have the compressor tested at the same time.