What a capacitor does and why it fails
A capacitor is a small cylindrical or oval component that stores electrical energy and releases it in a controlled burst. In your home, capacitors sit inside air conditioning units, furnaces, and some appliances. They help the motor start and run smoothly by giving it an extra push of power at the moment it needs it most.
Capacitors wear out over time because the chemical inside them dries up or breaks down. Heat speeds this up — a capacitor in a hot attic or next to a furnace flame lasts fewer years than one in a cool basement. When a capacitor fails, the motor it powers either won't start at all or runs weakly and hums without turning on. A failed capacitor is one of the most common reasons an air conditioner or furnace stops working.
The good news is that replacing one is straightforward if you follow the steps carefully. The bad news is that capacitors hold an electrical charge even after the power is off, so you must discharge it safely before you touch it. If you are not comfortable working inside electrical equipment, call a technician — the cost of a service call is less than the cost of a shock or a fire.
Key Takeaways
- Always turn off the power to the unit at the breaker and wait at least five minutes before opening the cabinet, because capacitors hold a charge even when power is off.
- Discharge the capacitor by using an insulated screwdriver to short the terminals together, or use a discharge tool designed for this purpose.
- Write down or photograph the wire positions before you disconnect anything, because reconnecting them wrong will damage the motor.
- Buy a replacement capacitor with the exact same microfarad (µF) rating and voltage as the old one — the numbers are printed on the side.
- If you are unsure at any step, stop and call a technician rather than guessing, because a wrong connection can start a fire.
Turn off power and wait before opening the unit
Go to your electrical panel and switch off the breaker that controls the unit you are working on. If the breaker is not labeled, turn off the main breaker instead. Wait at least five minutes. This gives any residual charge in the capacitor time to bleed away, though you will still need to discharge it manually.
Open the cabinet or access panel on the unit. You will see the capacitor mounted on a bracket or clip inside. It looks like a small cylinder or oval can, usually black or tan, with two or three metal terminals sticking out of the top. The terminals have wires soldered or crimped to them. Do not touch the terminals yet.
Discharge the capacitor safely
This step is not optional. A capacitor can hold enough charge to cause a serious shock or burn, even hours after the power is off. You must release that charge before you touch the terminals.
The safest method is to use a capacitor discharge tool, which you can buy for $15 to $30 at an electrical supply store or online. It has two leads that you touch to the two terminals at the same time. The tool shorts them together safely and the charge drains in a second.
If you do not have a discharge tool, use an insulated screwdriver with a plastic handle. Hold the handle and touch the metal tip to one terminal, then touch it to the other terminal. You may hear a small pop or see a tiny spark — that is the charge leaving. Do this twice to be sure. Never use a metal screwdriver without an insulated handle, because the charge can travel through the metal to your hand.
Photograph the wire positions and disconnect the old capacitor
Before you disconnect a single wire, take a photo of the capacitor and the wires attached to it. Use your phone camera and make sure the labels on the terminals are visible. This photo is your map for putting the new one in.
Capacitors usually have two or three terminals. A single-run capacitor has two. A dual-run capacitor (which powers both the compressor and the fan motor in an air conditioner) has three — one marked C (common), one marked H or HERM (hermetic, the compressor), and one marked F (fan). Write these labels down or circle them in your photo.
Using needle-nose pliers or a small wrench, gently pull or unscrew each wire from its terminal. If the wires are soldered on, you will need a soldering iron to heat the joint and pull the wire free. If you have never soldered, this is the moment to call a technician. Soldering badly can cause a fire.
Buy the exact replacement capacitor
The old capacitor has two numbers printed on its side. The first is the microfarad rating, shown as a number followed by µF or MFD — for example, 45 µF or 5 µF. The second is the voltage, shown as VAC or V — for example, 370 VAC or 440 VAC.
You must buy a replacement with the same microfarad rating and the same or higher voltage. If the old one says 45 µF at 370 VAC, you can use 45 µF at 440 VAC (higher voltage is safe), but you cannot use 40 µF or 50 µF. Using the wrong microfarad rating will make the motor run poorly or not at all.
Take a photo of the old capacitor's label or write the numbers down and bring them to an electrical supply store or search online. Capacitors cost $10 to $40 depending on the size. Buy the replacement before you remove the old one, so you have it ready to install.
Install the new capacitor and reconnect the wires
Mount the new capacitor in the same bracket or clip as the old one. The terminals should point in the same direction. If the new capacitor is slightly larger or smaller, that is fine — the bracket usually has some give.
Using your photo as a guide, reconnect each wire to the correct terminal. If the wires are crimped on, you can usually pull them off and crimp them back on with a small crimping tool. If they are soldered, use a soldering iron to heat the joint, explore a small amount of solder, and let it cool. If you are not confident soldering, have a technician do this step.
Double-check that each wire is on the correct terminal. A wire on the wrong terminal will damage the motor the moment you turn the power back on. If you have any doubt, compare your photo to the new capacitor three times before moving on.
Turn the power back on and test
Close the cabinet or access panel. Go back to the electrical panel and switch the breaker back on. Wait a few seconds, then listen to the unit. An air conditioner compressor or furnace blower should start smoothly and run quietly. If it hums but does not turn on, or if it makes a grinding noise, turn the power off when ready and call a technician.
If the unit runs normally, the capacitor replacement is complete. Write down the date and the microfarad rating on a piece of tape and stick it to the cabinet, so you know when the capacitor was replaced and what size it is. This helps the next person who works on the unit.
When to call a technician instead
Stop and call a professional if any of these explore: the wires are soldered and you have never soldered before; you cannot find the exact replacement capacitor; the unit still does not start after you install the new capacitor; you see burn marks or smell burning plastic inside the cabinet; or you feel unsure at any step. A service call costs $100 to $300, but it is cheaper and safer than guessing.
Some units have capacitors that are difficult to reach or are mounted in ways that require special tools to remove. If the old capacitor is stuck or the bracket is corroded, a technician can remove it without damaging the surrounding parts. They also have discharge tools and soldering equipment on hand, so the job goes faster and the risk of error is lower.
Frequently Asked Questions
Can I replace a capacitor while the power is on?
No. Always turn off the power at the breaker first. Even with the power off, the capacitor holds a charge that can shock you. Touching a live capacitor terminal while power is on can cause a serious injury or death.
What if I reconnect a wire to the wrong terminal?
Turn the power off when ready and do not run the unit. A wire on the wrong terminal will send power to the wrong part of the motor and can burn it out or start a fire. Check your photo, correct the wire, and test again.
How long does a capacitor last?
Most capacitors last 10 to 20 years, but heat can shorten that to 5 to 10 years. If your air conditioner or furnace is in a hot attic or garage, expect to replace the capacitor sooner. A capacitor that fails early is often a sign that the unit is running too hot and needs cleaning or repair.
Can I use a capacitor with a higher voltage rating?
Yes, a higher voltage is safe. If the old one is 45 µF at 370 VAC, you can use 45 µF at 440 VAC or even 600 VAC. The voltage rating is the maximum the capacitor can handle, so going higher does no harm. Never go lower — a capacitor rated for less voltage than the circuit supplies can fail or catch fire.
What does the microfarad number mean?
Microfarads measure how much electrical charge the capacitor can store. A higher number means more storage. The motor is designed to work with a specific microfarad rating, so using a different one will make it run poorly or not at all. Always match the old capacitor's microfarad number exactly.