What a capacitor does and why it fails
A capacitor in your heat pump is a cylinder that stores electrical energy and releases it to start the compressor and fan motors. When a capacitor fails, your heat pump will not start, or it will run the fan but not the compressor—meaning no heating or cooling. A bad capacitor is one of the most common heat pump failures and one of the cheapest to fix if you catch it early.
Capacitors wear out because they cycle thousands of times a year. Heat, humidity, and power surges all shorten their lifespan. A capacitor that is five to ten years old is more likely to fail than a new one, but age alone does not may provide failure. The first sign is usually a humming sound from the outdoor unit with no compressor activity, or a burning smell near the unit.
You can test a capacitor yourself with a multimeter, but the test only tells you whether the capacitor is holding a charge—not whether it will work correctly under load. A technician's test is more thorough, but learning to read a multimeter gives you useful information to share with a repair person or to decide whether to call one at all.
Key Takeaways
- A capacitor stores electrical energy to start your heat pump's motors; when it fails, the compressor will not run even if the fan does.
- You can test a capacitor with a multimeter set to the ohms or capacitance setting, but the test does not measure real-world performance under load.
- Before testing, turn off power to the heat pump at the breaker and discharge the capacitor by touching a screwdriver across its terminals to prevent electric shock.
- A reading of zero ohms or no capacitance value usually means the capacitor is dead; a very high reading means it is open and not storing charge.
- If your test shows a bad capacitor, a technician can replace it in under an hour for a fraction of the cost of a new compressor or motor.
Safety steps before you touch anything
A capacitor holds an electrical charge even when power is off. Touching the terminals without discharging it first can cause a painful shock or damage your multimeter. This is not a theoretical risk—it happens to homeowners and technicians every year.
First, turn off power to the heat pump at the circuit breaker. Wait five minutes. Then locate the capacitor in the outdoor unit—it is a metal cylinder about the size of a soda can, usually mounted near the compressor. Look for two or three wire terminals on top.
Use an insulated screwdriver to touch both terminals at once, holding the screwdriver by the handle. You may see a small spark; that is the capacitor discharging and is normal. Do this twice to be sure. Now the capacitor is safe to test. If you are not comfortable doing this, stop here and call a technician.
Testing with a multimeter on the ohms setting
Set your multimeter dial to the ohms (Ω) setting. If your meter has multiple ohm ranges, start with the highest one. Touch one probe to each capacitor terminal. The needle or digital display should move toward zero ohms, then climb back up toward infinity as the capacitor charges through the meter.
If the reading stays at zero or does not move at all, the capacitor is shorted and dead. If the reading stays at infinity and never moves, the capacitor is open and cannot hold a charge. Either result means the capacitor has failed. A healthy capacitor will show that climbing pattern—zero to infinity—because the meter is watching the capacitor charge.
This test is quick and works on most multimeters, but it does not tell you the actual capacitance value. It only tells you whether the capacitor can store and release charge at all. A capacitor that passes this test might still be weak and fail under the stress of starting a motor, but it is a useful first check.
Testing with a multimeter on the capacitance setting
If your multimeter has a capacitance (F or µF) setting, you can measure the actual stored charge. First, discharge the capacitor again with the screwdriver. Set the dial to the capacitance setting and touch one probe to each terminal.
The display will show a number in microfarads (µF). Look at the capacitor's label—it will list a rating like "45 µF" or "5 + 45 µF" (dual capacitors have two ratings). Your reading should be within 10 percent of that number. If your reading is much lower—say, 30 µF when the label says 45 µF—the capacitor is weak and losing capacity.
A reading of zero or a value far below the label means the capacitor has failed. This test is more precise than the ohms test, but not all multimeters have a capacitance setting. If yours does not, the ohms test is sufficient to tell you whether the capacitor is completely dead.
Understanding dual capacitors and single capacitors
Some heat pumps use a dual capacitor, which looks like one cylinder but contains two capacitors inside. The label will show two ratings separated by a plus sign, like "5 + 45 µF." The first number runs the fan motor; the second runs the compressor. A dual capacitor has three terminals: one common terminal and one for each motor.
If you are testing a dual capacitor, test between the common terminal and the fan terminal first, then between the common terminal and the compressor terminal. Each should read close to its labeled value. If one side is dead and the other is fine, the whole dual capacitor must be replaced—you cannot replace half of it.
A single capacitor has two terminals and runs only the compressor or fan. It is simpler to test: just touch the probes to both terminals. Single capacitors are less common in modern heat pumps but are still found in older units.
What to do if the capacitor is bad
If your test shows a dead or weak capacitor, you have found the problem. A new capacitor costs between $15 and $50 depending on the type, and a technician can install it in 20 to 30 minutes. This is one of the cheapest repairs a heat pump can need.
Before you call a technician, write down the capacitor's label information—the µF rating, voltage (usually 370V or 440V), and whether it is single or dual. This helps the technician order the right part and confirms you have diagnosed the problem correctly. If you tested it yourself and found it bad, tell the technician what you found; they will verify it but will appreciate the information.
Do not try to replace the capacitor yourself unless you have electrical experience. The discharge risk is real, and installing the wrong capacitor can damage the motor or compressor. A technician will also check the motor and compressor to make sure the bad capacitor did not cause other damage.
When the test shows the capacitor is good
If your multimeter test shows the capacitor is holding charge and reading close to its label value, the capacitor is probably not the problem. This does not mean your heat pump is fine—it means the failure is elsewhere, likely in the compressor, motor, or control board.
A compressor or motor failure is more expensive than a capacitor replacement and usually requires a technician to diagnose. If the capacitor tested good but your heat pump still will not run, note what you found and call a technician. You have already ruled out one common cause, which saves time and narrows the search.
Frequently Asked Questions
Can a capacitor fail partially and still work?
Yes. A capacitor can lose capacity over time and still technically work, but it will struggle to start the motor. You might hear a humming sound or notice the compressor takes longer to start. If your capacitance reading is 20 percent or more below the label, the capacitor is weak and should be replaced soon.
What does a burning smell near the heat pump mean?
A burning smell usually means the capacitor has failed and the motor is trying to start without enough electrical push. Stop running the heat pump and call a technician. Running it further can damage the motor. A burning smell is not always the capacitor, but it is a sign something electrical has failed.
Is it safe to replace a capacitor myself?
Replacing a capacitor requires disconnecting wires, discharging the old one, and installing a new one with the correct voltage and µF rating. If you make a mistake, you can damage the motor or shock yourself. Unless you have electrical experience, hire a technician. The labor cost is small compared to replacing a motor.
How long does a capacitor usually last?
Most capacitors last five to ten years, but some fail sooner in hot climates or after power surges. There is no way to predict failure, which is why testing is useful. A capacitor that is ten years old is more likely to fail soon, so replacing it before it dies can prevent a breakdown during peak heating or cooling season.
Can I use a capacitor with a higher voltage rating?
Yes, you can use a higher voltage rating—for example, a 440V capacitor in place of a 370V one. You cannot use a lower voltage rating. However, always match the µF rating exactly. Using the wrong µF rating will damage the motor or compressor, even if the voltage is correct.