Updated: 10/16/2026
Keeping your home cool during the summer is important, but there are many issues that can prevent your air conditioning from functioning effectively or stop it from working at all. A failing or blown-out AC capacitor is one such issue. The capacitor helps each of the system’s motors start, and if it fails, your AC won’t run. Replacing one is a common AC repair, but it involves stored electrical charge and high voltage, so it is not the right job for every homeowner. Here is how to spot the warning signs of a bad AC capacitor, what a humming or non-starting AC means, how replacement works, and when to call a professional for AC repair in Bountiful, UT.
The Function of an AC Capacitor
The various parts of your air conditioning system require a huge amount of power to start up—typically far more power than the circuit can provide. The AC capacitor works by storing a powerful charge of electricity so that the AC compressor, outside fan motor and indoor blower fan motor can start up without tripping the circuit breaker. When any of the motors need to start, the capacitor releases the energy and basically jump starts the motor. Some systems have a single capacitor that powers all three motors, while others have a separate capacitor for each of the three motors.
Signs of a Bad AC Capacitor
Capacitors typically fail due to normal wear and tear. Over time, the capacitor loses its ability to hold a charge and this can lead to several issues. AC capacitors can also be damaged by a major power surge. When the capacitor starts to fail, you may notice one or more of these warning signs:
- Humming or buzzing when the AC tries to start. If the capacitor has fully failed, the motor won’t turn on at all and will make a humming or buzzing sound whenever it tries to start.
- Loud clicking noise. A failing capacitor will often click loudly. Act quickly, because the motor it powers could burn out.
- Hard starting. The motor struggles to turn on and then immediately shuts off again.
- The AC shuts off on its own or won’t stay running. The system starts, then stops, or turns on and off repeatedly.
- Weak or warm air from the vents. If the fan or compressor isn’t getting the boost it needs to run properly, your home won’t cool the way it should.
- A swollen, bulging, or leaking capacitor. Only look with the power off, and never touch it.
These symptoms can also point to other AC problems, such as a failing motor or a faulty contactor, so a technician should confirm the cause. If your system keeps starting and stopping, you may also want to read our guide on reasons why your AC is short cycling.
AC Humming but Not Starting? Here’s What It Means
If your air conditioner hums or buzzes but the outdoor fan or compressor won’t start, a bad AC capacitor is one of the most common causes. The motor is receiving power, but it isn’t getting the jolt of energy it needs to begin turning.
If this happens, set the thermostat to OFF and switch off the AC at the breaker instead of letting the system keep trying to start. Repeated start attempts can put extra strain on the motor. A humming AC can also be caused by a failed motor, a worn contactor, or a wiring problem, so a certified technician can test the capacitor and the rest of the system to find the real cause.
AC not starting or making a humming noise? Don’t wait for the heat to build. Call Blue Best at (385) 300-8391 or schedule AC repair online, and our certified technicians will diagnose the problem and get your cooling back on track.
Capacitor Replacement Warnings: When to Stop and Call a Pro
Replacing a capacitor can look simple, but a mistake can lead to injury or a bigger repair bill. Keep these warnings in mind:
- Stored electricity is dangerous. A capacitor can hold a powerful charge even after the power is shut off. Never touch the terminals or wires until it has been safely discharged.
- Shut off the power first. Set the thermostat to OFF and shut off power to the unit before opening the access panel.
- The replacement must match. Use a capacitor with the same specifications as the original. The wrong part can leave your AC not starting or damage the motor it powers.
- Photograph and label the wires. Wires connected to the wrong posts will keep the system from running.
- A new capacitor may not be the fix. If the new capacitor doesn’t solve the problem, or it fails again soon after, something else is likely wrong, such as a burnt-out motor.
Call a certified AC technician instead of doing it yourself if you aren’t comfortable working near high-voltage electrical parts, if you see burn marks, melted wires, or a leaking capacitor, or if the breaker keeps tripping.
Steps for Removing an AC Capacitor
Replacing an AC capacitor is definitely something you can do yourself, but you need to be extremely careful when doing so. The capacitor contains a huge amount of electricity and can give you a major shock if you touch it without first dispersing the energy. This can be done by taking an insulated screwdriver and placing the blade over the two terminals. The metal blade creates a circuit between the two terminals, which discharges the stored electricity.
Safety note: Many technicians use an insulated discharge tool with a built-in resistor instead of a bare screwdriver, because shorting the terminals directly can cause sparks and damage the capacitor. If you don’t have the right tools or experience, leave this step to a professional.
Before discharging the capacitor, it is essential that you shut off the power to the unit. First set the thermostat to “OFF” and then shut off the electricity to the unit. Some systems have a circuit breaker or fuse box outside near the AC compressor. The circuit breaker can be simply switched to off, but if you have a block fuse, this will need to be removed.
If there is no outside power shut-off, you will have to switch off the power to the unit by flipping the AC circuit breaker in your main electrical panel. It will be a two-handle, 240-volt breaker and it should be labeled as air conditioner. If the breaker isn’t labeled, you can either try to flip each 240-volt breaker until you find the correct one or simply shut off the main circuit breaker that controls the entire electrical system.
Once the power is off, remove the access panel near the back of the unit where the power feeds into it. You should see a metal cylinder that looks similar to a can with wires coming out of the top or bottom, and this is the capacitor. Take your insulated screwdriver and discharge the capacitor.
You’ll then need to label the wires before removing them. Each wire has to go to its specific post or else the capacitor won’t work so you need to label them so you know where each wire goes when installing the replacement part.
If you have a dual capacitor that powers the blower fan motor inside, it will have three wires—fan, compressor and common. If the capacitor only powers the compressor and outdoor fan motor, it will only have two wires. There should be markings on the capacitor that show what each wire is for. The fan wire should be labeled “F”, the common wire will be labeled “C” and the compressor wire is usually labeled “H” for hermetic. Once you’ve labeled the wires, you can disconnect them and remove the capacitor.
Take the old capacitor with you to the home improvement or hardware store to ensure you get the right part. The new capacitor will need to have the exact same specifications or else it won’t work. The three important specifications are tolerance, capacitance and load voltage. The new capacitor may be a slightly different size or not look exactly like the old one, but it will still work as long as these three specifications are the same.
Installing the New AC Capacitor
Once you have the new capacitor in hand, it’s time to get it installed. Secure the new capacitor in place using the metal mounting strap. If the old strap is damaged, you will need to get a new one. After mounting the capacitor, slide the end of each wire connector onto the post. Make sure that each wire goes to the correct post.
The new capacitor should be ready to go, but you’ll want to test to make sure it works before closing the unit back up. Switch the power back on to the system by flipping the circuit breaker or reinserting the block fuse. Next, turn your thermostat to cooling mode and set the temperature to lower than the current indoor temp so the system turns on.
If the system immediately turns on and stays on then you’re all set. If it doesn’t turn on, then most likely you didn’t connect the wires to the correct post. In this situation, you will need to shut the power off again and then check to make sure that each wire is in the correct location. Make sure to discharge the capacitor again before touching it. Even if it is not wired correctly, it will most likely still have a charge stored and could shock you if you don’t discharge it first.
If the capacitor is wired correctly and your air conditioning still won’t turn on, then you either didn’t buy the correct capacitor or the system has some other issue that is preventing it from starting. In this case, you will need to have a certified AC technician inspect the system to determine what is causing it not to start. Most likely the problem is that one of the motors is burnt out and needs to be replaced.
Expert AC Capacitor Replacement and AC Repair in Bountiful, UT
If you suspect your AC capacitor has failed and aren’t comfortable with replacing it yourself, the certified HVAC technicians at Blue Best Plumbing, Heating, Air, & Generators are here to help. We can also inspect your entire AC system to see if there are any other issues that are preventing it from starting or working as efficiently or effectively as it should. We service and repair all types of central air conditioners from any brand. If your AC compressor has died or you’re looking to upgrade to a more energy-efficient unit, we also specialize in air conditioning installation.
We have more than two decades of experience providing air conditioning and heating services to residents of Bountiful and the rest of the Greater Salt Lake City area, and we also specialize in indoor air quality services.
Don’t let a failed capacitor leave you without cooling. Call Blue Best at (385) 300-8391 or schedule your AC repair online today. Regular AC maintenance can also help catch worn parts like capacitors before they fail.
