What an AC capacitor does and why it fails
A capacitor is a small cylindrical or oval component inside your air conditioning unit that stores electrical energy and helps start the compressor and fan motor. When a capacitor fails, your AC will not cool, or it will run but the compressor or fan will not start — even though power reaches the unit. A failed capacitor is one of the most common AC repairs, and replacing one costs far less than replacing a compressor or the entire system.
Capacitors wear out because they cycle on and off thousands of times per cooling season. Heat, humidity, and power surges all shorten their lifespan. Most capacitors last 10 to 20 years, though some fail much sooner if the unit runs constantly or if electrical stress is high. A capacitor that has reached the end of its life will not store charge properly and cannot start the motor, leaving you without cooling.
You may notice a burnt smell near the outdoor unit, a clicking sound when the AC tries to start, or the compressor humming but not running. These are signs the capacitor is failing or has failed. Some capacitors swell visibly or leak fluid before they stop working entirely.
Key Takeaways
- A capacitor is a small electrical component that starts your AC compressor and fan; when it fails, the AC will not cool even if power reaches the unit.
- Capacitor replacement costs between $150 and $400 for parts and labor, depending on the capacitor type and your location.
- You can have a technician replace it, or if you have electrical experience, you can buy the part yourself and install it — but the capacitor must match the microfarad (µF) rating and voltage of the original.
- Most capacitors last 10 to 20 years; heat, humidity, and power surges shorten that lifespan.
- A failed capacitor is not a safety hazard to your home, but a capacitor that is swelling, leaking, or burnt should not be left in the unit.
Signs your capacitor has failed
The most obvious sign is that your AC will not turn on or will not cool, even though the thermostat is set correctly and the breaker has not tripped. You may hear a clicking or buzzing sound from the outdoor unit when you turn on the AC — this is the compressor or fan trying to start but unable to because the capacitor cannot deliver the initial electrical push.
A humming sound without the compressor or fan actually running is another strong indicator. The motor is receiving power but the capacitor is not giving it the jolt needed to overcome inertia and begin spinning. If you smell a burnt or chemical odor near the outdoor unit, the capacitor may have overheated or shorted internally.
Visually, a failed capacitor may be swollen, cracked, or leaking a dark oily fluid. If you see any of these signs, the capacitor should be replaced. Do not ignore a leaking or swollen capacitor — it will not recover, and leaving it in place risks damage to other electrical components.
How to identify the right replacement capacitor
Before you buy a replacement, you need to know the microfarad (µF) rating and voltage of your current capacitor. This information is printed on the capacitor itself — look for a label that reads something like "45 µF 370V" or "10 µF 250V". The microfarad rating tells you the capacitor's storage capacity; the voltage tells you the electrical pressure it can handle.
Write down both numbers exactly. Buying a capacitor with the wrong microfarad rating will not start your motor correctly — it may not start at all, or it may start but run poorly and overheat. Buying one with too low a voltage rating risks the capacitor failing again quickly or catching fire.
If the label is worn or illegible, you can look up your AC model number (usually on a plate on the outdoor unit) and search for the capacitor specifications online, or you can call an HVAC technician to confirm the rating before you order. Many capacitors are inexpensive — $15 to $50 for the part itself — so it is worth getting it right the first time.
Replacing the capacitor yourself versus hiring a technician
If you have experience working with electrical components and feel comfortable working inside an AC unit, you can replace a capacitor yourself. The process is straightforward: turn off power to the unit, discharge the capacitor (which holds a charge even when power is off), disconnect the wires, remove the old capacitor, and install the new one in the same position with the wires in the same terminals.
However, capacitors hold an electrical charge that can cause a painful shock even after the unit is powered down. If you are not confident discharging it safely, or if you are unsure which wires go where, hire a technician. The labor cost is usually $100 to $300, and the technician will also verify that the capacitor is the only problem — a compressor or fan motor failure will look similar but requires a different repair.
Most homeowners choose to hire a technician because the risk of electrical shock and the cost of a mistake (damaging the motor or other components) outweigh the savings of doing it yourself. A technician can also check whether the capacitor failure was caused by a larger electrical problem, such as a power surge or a failing compressor that was drawing too much current.
What to expect from a professional replacement
When you call an HVAC company, describe the symptoms — no cooling, clicking sounds, or a burnt smell — and mention that you suspect a capacitor failure. The technician will visit, confirm the capacitor is the problem, and replace it on the spot if they have the right part in their truck. Most common capacitor ratings are stocked by service technicians, so replacement often takes less than an hour.
The total cost is typically $150 to $400, depending on your location, the time of year (summer calls cost more), and whether the technician has to order a less common capacitor. Some companies charge a diagnostic fee ($50 to $100) that is credited toward the repair if you proceed. Ask about this before the technician arrives.
After replacement, the AC should cool normally. The technician will run the unit to confirm it is working and may check the voltage and amperage to may support the new capacitor is performing correctly. If the AC still does not cool after the capacitor is replaced, the problem is something else — a refrigerant leak, a failed compressor, or a blocked condenser — and will require further diagnosis.
Capacitor failure and your warranty
If your AC system is still under warranty, check whether capacitor replacement is covered. Most manufacturer warranties cover the compressor and major components but not the capacitor, which is considered a wear item. However, some extended warranties or service plans do cover capacitors.
If your unit is out of warranty, the replacement is an out-of-pocket expense. Capacitor failure is not caused by a manufacturing defect in most cases — it is normal wear — so you cannot usually return the unit or claim it under a product defect.
Preventing early capacitor failure
You cannot prevent a capacitor from eventually wearing out, but you can reduce the risk of premature failure. Keep the outdoor unit clean and free of debris so it does not overheat. Make sure the unit has adequate airflow and is not in direct sunlight all day. Have your AC serviced annually — a technician can check the capacitor's voltage and amperage to spot a failing capacitor before it stops working entirely.
Power surges from lightning or utility problems can damage a capacitor when ready. If you live in an area with frequent thunderstorms, consider installing a whole-home surge protector on your electrical panel. This will not prevent all capacitor failures, but it will reduce the risk from external electrical events.
Frequently Asked Questions
Can I run my AC if the capacitor is bad?
No. If the capacitor has failed completely, the compressor or fan will not start, so the AC will not cool. If the capacitor is failing but not completely dead, the AC may run but cool poorly or cycle on and off. Running the AC with a failing capacitor can damage the compressor, so it is best to replace the capacitor as soon as you notice symptoms.
How do I know if it is the capacitor and not something else?
A clicking or humming sound from the outdoor unit when you turn on the AC is a strong sign the capacitor has failed. If the AC does not cool and you hear no sound at all from the outdoor unit, the problem may be a tripped breaker, a blown fuse, or a failed compressor. A technician can test the capacitor with a multimeter to confirm it is the problem.
What happens if I install the wrong capacitor?
If the microfarad rating is wrong, the motor may not start or may start but run poorly and overheat. If the voltage is too low, the capacitor may fail again quickly or overheat. Always match the microfarad and voltage ratings exactly. If you are unsure, ask a technician or the parts supplier to confirm before you install it.
Is a capacitor failure dangerous?
A failed capacitor is not a fire hazard to your home, but a capacitor that is swollen, leaking, or burnt should be replaced promptly to avoid damage to other components. A capacitor that is actively leaking can corrode nearby wires and terminals. The electrical charge inside a capacitor can cause a painful shock, so do not touch it without discharging it first.
How often do capacitors need to be replaced?
Most capacitors last 10 to 20 years. If your AC is older than 15 years and has never had a capacitor replaced, it may be worth replacing it as preventive maintenance, even if it is still working. This can prevent an unexpected failure during peak cooling season when technician availability is low.