Common Air Conditioner Capacitor Failure Symptoms

Capacitor Failure Symptoms are sometime harder to identify without the right tools and expertise. Capacitors can be found in several different sizes and shapes. There may be one or multiple capacitors included in your ac with regards to the design. While the most popular capacitors are the types found on circuit boards, the ones most associated with air conditioning will be the bigger capacitors that assist start and run your motors. There are several motors present in an conditioner and heater systems:

  1. Compressor Motor Run Capacitor: Here is the most popular capacitor that fails. Most of the time these are duel capacitors, which means that there are 2 capacitors built into one. A duel capacitor will have three terminals on the top where a single capacitor will simply have two terminals on top.
  2. Outside Fan Motor Run Capacitor: That is the smaller capacitor found with or part of the Compressor Motor Capacitor. It helps start and run the outside fan that blows air through the outside coils.
  3. Indoor Blower Motor Run Capacitor: Similar to the outdoor fan motor capacitor, it is a small, single (two terminals) capacitor that will help start and run the indoor blower motor.
  4. Start Capacitor: Some models include an auxiliary start capacitor that helps jump start the motors. Most are found on the compressor. These are less common.

Why do capacitors go bad

When capacitors go south, they either go out entirely or they decline in strength. The decline is measured in the “capacitance” units called micro farads. When the capacitors are produced, they are labeled with their specifications capacitance and a range +/- from the design capacitance. This range is the allowable operating range, usually labeled in a percentage range, for the capacitor. If the capacitor starts to fail, the capacitance measured will be outside of the labeled range. Sometimes these are difficult to find any symptoms unless you have the proper tools. Warning, there is a risk of severe shock, so do not try this at home.

Diagnosing a bad capacitor

If a capacitor goes out completely, or is severely out of its operating range, it may prevent the motor affiliated with it from running completely. If the compressor motor is not working, then the air conditioner will not cool. If the outside fan is not working the compressor may cause problems and will short cycle or cease working. If the indoor fan stops, you’ll find that there is no air blowing through the vents.

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268 thoughts on “Common Air Conditioner Capacitor Failure Symptoms

  1. Q

    I have a Goodman ac unit. The fan is blowing on top of the unit and a rowing noise will cut on and off. Warm air comes out of the vents. What could that be? Is it a easy fix?

    1. thehvacguy Post author

      Sounds like you may have problems with your compressor not running. I would check the amp draw on the compressor.

  2. Ted

    I appreciate all the info here but so far have not found my particular issue. Back in March I turned off my heat pump and was using window fans as it was rather nice here for a long time. And I wanted to save some money as well. In June it started to heat up here pretty good and I went to turn on the unit to cool things down. Inside handler fan came on in normal time. Did not think much of it until I noticed no cold air from vents about 20 minutes later. Went out side to check the unit to see if it kicked on. Has had problems in the past with blowing out capacitors. The top fan was running fine and quite like normal but no sound of compressor running. Killed power to the unit and waited a few minutes then turned back on to see if it would kick start. No luck but low hum under fan can be heard like compressor is stuck. Went and bought a window unit for the short term as I could not afford to call out a service. Well I finally got around to getting a replacement capacitor to swap out and to test the unit. After installing same, no compressor. Could it be the relay or would you think that the compressor is just dead? Unit is over 15 years old so I am guessing I will need to belly up and replace the system.

    1. thehvacguy Post author

      It sure sounds like your compressor is dead. There are two tests to tell for sure, one is to measure the amp load on start. If it exceeds teh RLA rating on the name plate, you are dealing with a locked rotor and the compressor needs to be replaced. If no amps are measured, then you should check the resistance of the windings. Also check connectivity from each winding to ground. If the resistance on the windings is o or OL, then you have a short or open winding. replace the compressor. If the connectivity to ground is found, replace the compressor.

  3. Jacob

    I have an old heat pump and yesterday when i got home from work i noticed the fan had stopped spinning. Now, i am not very knowledgeable about hvac systems, but i did some research and i think it may be a bad capacitor (when i turn on the A/C the fan does not spin but it sounds like everything is running, after about 30-60 seconds it shuts off). But when i opened up the panel i found there was no cable attach for “fan”. Is this common?

  4. Danielle

    My central air stopped working. I own a lennox. The fan and the compressor are not running. I can get the fan to start with a push but the compressor does not kick in. Could this be a bad capacitor effecting the compressor and stopping it from starting?

    1. gordon williams

      Exact same problem..except when i changed capacitor it worked for roughly 24.hours….then it stopped again.
      This time i cant get the fan to spin and.i still.get the humming sound but no compressor or fan action.
      Inside blower working just.fine on fan and heat. What could be the issue?
      What could blow.that fast?,

      1. thehvacguy Post author

        Well it may not be the capacitor now. Running on a bad capacitor can damage the other motors. Check the capacitance. Also check the amp draw on the motors as they start. You could have damaged motors on both the fan and compressor. Prevention is the best cure. An annual maintenance program can help prevent major motor failure.

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