What is an AC Capacitor?

Depending on where you live in the country, you have heard people talk a lot about your air conditioning system run capacitor or what others may call your start capacitor. Some regions of the U.S. tend to have more failed fan motors, and other areas have an issue with leaking refrigerant coils. Below, we will talk about the portion of your air conditioner affected when you have a bad capacitor.

Always remember that you should consult a qualified HVAC technician before attempting to test your ac capacitor.

What Does an AC Capacitor Do?

capacitor stores and releases electrical energy during every cycle change. A run capacitor matches the system to create the ideal phase shift for optimal running efficiency and power usage by the motor.

Contrary to what many people say, a capacitor does not “boost” the voltage or create extra energy. The capacitor only stores and releases energy, which results in more or less phase shift and more or less capacitive reactance or “capacitance,” as we often refer to it.

Can you Test an AC Capacitor While the Unit is Running?

An experienced HVAC technician can test the AC capacitor while the unit is running by simply measuring the current of the fan motor start winding coming off of the capacitor and multiplying it by 2652. You will then divide the number you have by the voltage you are measuring across the capacitor.

Test AC Capacitor under load

5 Most Common Causes for an Air Conditioner Capacitor to Fail

Start Capacitor

If your start capacitor for the ac condenser’s motor ever fails, the outdoor unit will sound like it is running but the fan blade is not spinning. These symptoms occur because the compressor is running down inside of the condenser. 

This can shorten the life expectancy of the compressor and the fan motor since it is still sending voltage to the motor even though they are not operating correctly. This will in turn cause expensive repairs to the homeowners’ air conditioning system.

Outdoor Unit

Another type of ac capacitor symptoms most common with not cooling is when the entire outdoor unit does not run, even if it has power going to it. Oftentimes, these failed ac capacitor symptoms will result in the outdoor unit making something like a humming noise for a few seconds and then cutting off for a few minutes. Most of the time this repair to the system will solve the home cooling problems.

Air Conditioning Blower Motor

Your air conditioner blower motor may have bad capacitors if you have tested the voltage rating to the part, but it is still not blowing cold air. If the bad capacitor parts are replaced for the motors, and the motor is still not blowing air then the motor components themselves may be the problem.


The fourth sign of problems with your air conditioner’s ac compressor and the motor is when you install the replacement capacitor but the ac motors, fan, and compressor are still not spinning to blow any cold air. This is common if you have run the air conditioner for an extended period of time with the old capacitor prior to replacing the parts related to the ac capacitor symptoms.

You can cool the thing that is inside of the container that moves the air from the outdoor unit. The air normally cools this but you can use water as long as you have tested the voltage and it is safe to pour water over these items.

Dirty Condensing Coil

f your air conditioning system has a dirty condenser coil, the excess heat will cause your capacitor to fail. Capacitors will have a shortened lifespan if the capacitor can not stay cool. Most brands will keep the capacitor cool by circulating air around the capacitors at the same time as moving it over part of the coil. This movement of air will ensure the capacitor does not overheat and burst.

You as, the customer or homeowner, can help to avoid this repair by ensuring you have a company like Service Emperor come to your home twice per year to perform maintenance services to your heating and air conditioning equipment.

In Conclusion

To ensure your heating and air conditioning equipment’s capacitors last as long as possible, schedule your Total Comfort Club membership visit. The prices of these membership visits will be far less than the pricing it costs for us to come out for an emergency call on a Friday evening when your family is all in town for the extended holiday weekend! (We all know THAT is the only time things like this seem to happen!)

We truly value our customers and we only want the absolute best for you, so if you ever have any questions related to capacitors or how your furnace or air conditioner works, we are only a phone calls reach away!

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Tersh Blissett

CEO of Service Emperor and part-time HVAC Guru

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