Key Takeaways
- You’ll need a multimeter and a screwdriver when checking an AC capacitor; be sure to turn off the power completely.
- Capacitor failure signs include an AC not starting, making loud noises, or blowing warm air.
- Professional AC capacitor replacement averages around $300.
When it’s hot outside, you likely rely on your air conditioner to keep your home comfortable. Suddenly realizing that your AC is no longer cooling your home can be frustrating and anxiety-inducing as a homeowner.
Whether your AC is blowing warm air or isn’t functioning at all, one common culprit is the AC capacitor. This key component basically stores energy to start up the AC’s motor and keep it running efficiently. Most systems have multiple capacitors, including one in the outdoor condenser and another in the indoor furnace. When a capacitor fails, the AC system won’t start.
“The capacitor is a very common problem in air conditioners,” said Tim Alagushov, co-owner and chief operating officer of IRBIS Air, Plumbing & Electrical in San Jose, California. “In the summer season, at least 30% of the units we service have a capacitor issue.” Fortunately, you may be able to diagnose a capacitor issue by testing it yourself. You can also have your capacitor function checked as part of routine HVAC preventative maintenance.
Be aware that many AC capacitors carry a high electrical charge and can be dangerous if not completely disconnected from power before handling. If you lack electrical or HVAC experience, you may want to skip troubleshooting and call an AC repair pro.
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Signs of a Bad AC Capacitor
If your AC capacitor is failing, you will likely notice certain signs. Common symptoms include:
- AC won’t turn on: A bad capacitor means the air conditioning system lacks enough energy to start the fan motor.
- No cold air: If your system runs but only blows warm air, it may not have enough energy to start the condenser.
- Clicking or humming: Loud noises from your outdoor air conditioning unit may indicate that the fan motor is struggling to start.
- Random shut-offs: If your AC starts and stops frequently on its own and won’t run continuously, you may have a capacitor issue.
- Burnt smell near condenser: This could be from an overheated motor or a burnt-out or leaking capacitor.
- Swollen or leaking capacitor: These are signs you’ll only notice if you visually inspect the capacitor itself.
Always check the basics first. If your AC isn’t working, check your breaker panel to see if the breaker for your AC has flipped. You should also inspect your thermostat to ensure it’s functioning properly. If your breaker and thermostat are fine, then move on to checking your capacitor.
Tools You Need To Check an AC Capacitor
To check for AC capacitor failure, you will need just a few key tools:
- A voltage multimeter with a microfarad setting to test the capacitor’s voltage
- A screwdriver with a fully insulated handle to safely discharge the capacitor
- Rubber electrical safety gloves for protection
- Tape for marking wired connections
With these tools, you should be able to detect a charge and discharge the capacitor safely.
However, capacitors can store large amounts of electricity for weeks or months even after the system is turned off. Touching the wrong part with a metal tool or making other small mistakes can result in an electric shock or cause costly damage to other parts of the system. If you’re unsure about the process, call a professional instead.
How To Check an AC Capacitor
If you have the necessary tools and some basic electrical knowledge, you can follow these steps to safely inspect an AC capacitor for issues.
Step 1: Turn Off the Power
Electrical work always comes with the risk of electrical shock. Never perform electrical work, such as testing a capacitor, without first completely turning off the power to the system you’re working on.
First, turn off the system at the thermostat. Then, disconnect the power to your AC system at the circuit breaker. If you have an outdoor service disconnect box near your outdoor condenser unit, open the box and move the handle or switch to the off position.
Step 2: Access the Capacitor
Use a screwdriver to remove the access panel on your outdoor condenser unit.
You’ll typically see the capacitor near the compressor and fan motor inside the unit. It is a metallic, cylindrical component, similar in size to a soda can, with two or three terminals on top. It may be attached to the unit’s housing or a bracket.
If the capacitor is visibly bulging, leaking, burnt, or damaged in any way, do not touch it. These are signs of a faulty capacitor that could cause an electrical shock. In this case, it’s best to leave the job to a technician.
Step 3: Discharge the Capacitor
Next, you’ll need to discharge any stored electrical energy in the capacitor. Don your gloves and use a screwdriver with a completely insulated handle that’s separate from the metal shaft.
Capacitors have multiple terminals on top usually labeled “C” for common, “FAN” for the motor fan, and “HERM” for the compressor. Firmly hold the screwdriver handle and carefully touch the metal shaft across the C and FAN terminals to bridge them. Then, remove the screwdriver and repeat the process across the C and HERM terminals.
The terminals will discharge immediately upon contact and may spark. Be careful not to touch the shaft to the FAN and HERM terminals simultaneously, as this may cause damage.
With the capacitor discharged, you can safely remove it from the AC unit. Snap a photo of the capacitor wiring so you can reference it later if needed, and use a marker and tape to label the wires. Detach the wires and remove the capacitor from its bracket, if applicable.
Step 4: Test With a Multimeter
Set your multimeter to capacitance (µF or MFD). Connect the probes to the C and HERM terminals. A good reading will be within ±10% of the capacitance value or µF rating printed on the capacitor’s label.
If your reading is out of the ±10% range or if your multimeter displays “OL,” then your capacitor needs to be replaced. Only attempt your own replacement if you’re comfortable with electrical work. If not, call a licensed HVAC technician.
Can You Replace an AC Capacitor Yourself?
You can technically replace an AC capacitor yourself, but Alagushov doesn’t recommend it.
“Homeowners can order a new capacitor for $15, and with basic electrical knowledge, they can replace it,” Alagushov said. “The question is, do they want to take this risk to potentially damage the whole system? There is an electric shock hazard. Even if they do it safely, they may cause burnout of other parts like the contactor or control board because of improper installation. The other issue is that the provider may void the warranty,” he added.
DIY AC Capacitor Replacement
Pros
- Replacement parts are very inexpensive
- Repair only takes a few minutes
- Only common tools are required
Cons
- DIY repairs can void your HVAC warranty
- There is a risk of electric shock
- Choosing the wrong part can burn out more expensive components
How Much Does It Cost To Replace an AC Capacitor?
A replacement AC capacitor costs as little as $15, according to Alagushov. If you don’t own a multimeter, they can cost anywhere from $10 to $200, depending on the quality.
On the other hand, capacitor replacement is one of the most common jobs that HVAC technicians perform, and Alagushov said that professional AC capacitor replacement is relatively inexpensive, averaging around $300.
Next Steps
The AC capacitor is a small yet critical component of an AC system that can be easily overlooked. If you’re experiencing any signs of a faulty or failed AC capacitor, you can catch problems early before they get too serious by doing a simple voltage test.
Always turn off the power completely and use a high-quality multimeter to confirm the issue. You can also call an HVAC pro, and many companies offer same-day diagnostics. If you test your AC capacitor and the voltage is low, replace it with extreme caution, and only if you have the proper electrical knowledge.
“DIY replacement is not recommended,” Alagushov said. “It’s better to take time to find a trusted company with a good reputation and work with them.”
FAQs About Checking AC Capacitors
What happens if my AC capacitor is bad?
If your AC capacitor is bad, you may experience a number of HVAC symptoms. Your system may not turn on or blow cold air. You may hear loud clicking or humming noises from the outdoor unit. Your AC randomly starting and stopping can also be a sign.
A bad capacitor means an AC system that doesn’t function well or at all. You may notice higher energy bills. If you smell burning or see that the capacitor is swollen or leaking, then turn off the power to your system and call an HVAC technician immediately to diagnose the problem.
Can I test an AC capacitor without a multimeter?
Yes, you can test an AC capacitor without a multimeter. If you have basic electrical knowledge, you can connect a light bulb and battery to the capacitor to see if it provides power. However, this test is far less accurate. A multimeter is always the most reliable way to test for microfarads and confirm that your capacitor’s voltage is within the recommended range for its rating.
Make sure the multimeter you use is able to test for capacitance. Most multimeters offer this function, but some don’t.
How long do AC capacitors last?
AC capacitors last 10 years on average, but may last anywhere from five years to 20. Factors affecting AC capacitor lifespan include high operating temperatures and overheating, voltage surges from lightning or the power grid, and capacitor quality.
Your capacitor’s lifespan may be up when your AC system starts humming or clicking, randomly turning off, or blowing warm air. A faulty capacitor may also bulge or leak fluid.
Is it safe to test or replace an AC capacitor myself?
It is safe to test an AC capacitor yourself, but only do so if you have basic electrical knowledge and the right tools, including a high-quality multimeter and a well-insulated screwdriver.
DIY capacitor replacement is not recommended. Improper parts or small electrical mistakes can burn out your AC system, void your warranty, or result in higher repair costs than you would have incurred otherwise. Call a licensed HVAC pro for replacement.
What’s the difference between a start capacitor and a run capacitor?
The primary difference between a start capacitor and a run capacitor lies in their function. While both provide power to an AC system, the start capacitor delivers a large voltage to start the fan motor and get the system running. Once your AC is on, the run capacitor delivers a lower continuous voltage to keep it running.
