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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721 thoughts on “Common Air Conditioner Capacitor Failure Symptoms”

  1. Got a question – My ac unit fan works fine; but when the compressor tries to fire up the capacitor draws power from the line and my lights dim for about 2 seconds then repeats every 3 or 4 minutes. It is not engaging the compressor. A few days ago it was working flawlessly. When I shut the unit off I can hear the capacitor buzzing back at my indoor circuit breaker. Have any ideas? Does that sound like a bad cap?

    1. Its not your capacitor but your contactor buzzing. Could be severely pitted which can prevent the compressor from starting.

      1. Thank you for the information; and that is a good call on the pits on the breaker/rack. I had already ordered a new capacitor just in case – because of all the various problems and symptoms noted abroad the internet. Might want to also consider my symptoms as a capacitor fail because when I replaced it – the AC started working again. From what I could tell before replacing the cap – when all power was removed the capacitor was vibrating and humming also in the outside unit. While I know that capacitors do hum some while in use; this thing continued to hum and it just reminded me of bad wiring discharging. Thanks again for the reply!

  2. Our outside unit stopped working yesterday. When turned on it clicks and can hear soft humming and fan doesn’t start. I can get fan start manually but the unit never kicks on.

  3. Last summer my blower motor had a hard time getting going and would then spin very slowly. I switched out the capacitor for new one and it fixed the problem.

    Tonight when I came home from work I noticed a burning plastic smell and the blower fan is moving rather slowly.

    I am wondering if it could be my capacitor starting to go bad? If a capacitor is starting to go bad would a symptom be a slower-moving fan?

  4. Hi Hvacguy,

    Our AC started blowing hot air on the inside of the house and cold outside. We noticed that The prong on the terminal for the compressor on the capacitor appears burnt. We moved the wire another prong on the same terminal and it fell of on its own after a few minutes but the air was cool inside during the short period that it was on. Do you think that replacing the capacitor may fix the problem all together? Also our capacitor has 4 prongs on all terminals does that mean something?

    1. Each terminal on the capacitor is marked with F C and HERM. Proper placement of the wires is crucial. How many prongs on each terminal is not important. But some manufacturers use 4 on Common (C), 3 on Herm (H) and 2 or 1 on Fan (F) but some have 4 on all three. Crimp the wire connector so that it is tighter when you apply it so that it is assured to stay on. Replacing the capacitor is only necessary when it is operating outside the specified rating limits.

  5. 98 degree day the Ac was running for hours non stop heard a squealing noise and went out back and the fan in the unit was not spinning.

    Turned off the Ac checked that the fan spun easily turned the Ac back on and pushed the fan it worked for about 10 minutes than the fan quit. Tried that 3 or 4 more times to the same results than just turned off Ac to prevent more damage.

    Is this simply a fan capacitator or do I have bigger issues?

    1. Start with the capacitor. But note that running for a long time on a weak or dead capacitor can damage the compressor and fan motors.

  6. I installed new cond fan motor with five wires old fan had three installed red and black to contacts brown to capacitor did not use purple and yellow started unit it ran proper direction then made a sound so I cut off unit and restarted unit ran opposite direction I changed direction with leads provide started unit up it ran for while then overheated and now will not run comp still runs do you have any ideals did not change capacitor

  7. My AC capacitor blew off and all the wires are melted (looks like there was a small fire). The unit is still under warranty (was installed in 2006) and the company is proposing to replace the capacitor and the wires to fix the issue. Should I accept their proposal or ask for a new unit ? I’m worried that the motor (or something else) will fail soon and that it won’t be under warranty anymore.

  8. Posting this out of frustration. I live in Miamk, FL and its hot as hell lately here, record heat. Inside my house too. Spent $250 replacing capacitors and other electricals on AC with a tech, and another $250 cleaning the coil inside the house, same tech. Initially I noticed coil would leak on top filter after unit reached desired temperature. This was the initial claim that started this. Fixed electricals cleaned coil, several days later I noticed a drop leaking on the filter again while I was just taking a look at the coil out of curiosity. Told the tech about this and he came to check and tells me a drop every couple of.mins on the filter is normal since the air getting sucked in will dry it. This sounds stupid to me and I’m looking for another tech for a second opinion. My house takes like 5 hours to go from 79 to 75 and before dropping $500 on this tech, the unit seemed to work better when it was dirty and the electricals were not swapped yet.

    1. I feel your pain. Unfortuately there is not a lot to go on here. He may have found issues with those parts. He should have checked to make sure the unit was working properly after the parts were replaced.

  9. Replaced the capacitor for the blower motor this morning because the system was buzzing but not starting the motor (which spins freely by itself when turned by hand). It worked for half a day and then I’m back to buzzing at the blower for about 60 seconds every 5 minutes or so and then nothing. And to make matters more frustrating, it randomly kicked on just fine at one point. Checked the circuit board and all the connections look fine (no blown solder points). Any thoughts?

    1. Sounds like a PSC motor with a capacitor. In that case I would be checking the fan switch or fan control board. Then also check the fan motor. Any one of those could be the root cause.

  10. Just replaced our contactor, and the start compositor as well as the transformer on a 5 ton split unit by carrier. Notices that my house is not cooling as before these parts was replaced. Some suggestions is that the compositor could be bad. Everything runs but the air is not as cold coming inside the house. The compressor and panel is cold on outside unit. The thermostat dose not go below 80 in the day time. It takes it a while to cool to 73 at night.

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