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Problem: What to look for when a capacitor is bad? My unit will not do anything!

Bad oval capacitor on the left

Problem: What do I look for when a capacitor is bad? My Air Conditioner unit will not do anything!

Answer: Capacitor problems are probably the second most common problem that I see every summer in our 28 year old HVAC business! Air Conditioning and heat pump systems low on refrigerant charge are the most common problem that I see. Changing out a capacitor is a simple, inexpensive problem that most all homeowners can fix easily if you are good with your hands and have the right capacitor. Weak or bad capacitors can also cause blower motors to not run or to run slow. We have two real good YouTube Videos near the bottom of this page which explains how to replace and troubleshoot an air conditioner’s capacitors.  We sell over 80 different capacitors on the following page: Please click here to see the capacitors we sell.

What to look for when a capacitor is bad:

The Air conditioner outdoor unit will not come on. Either the outdoor fan does not run, the compressor does not run, or both the fan and the compressor do not run. You have checked and reset your circuit breaker. The outdoor unit still does not come on. You can here a little humming sound inside the unit which is the low voltage contactor humming. Please pull the disconnect and disconnect the power to your outdoor air conditioning unit.

*Please make sure your electrical power is off before working on any air conditioning equipment. You take the door or cover off your outdoor unit’s control box and find a bad, swollen run capacitor. EPA stopped allowing manufacturers to produce capacitors with cancer causing PCB’s. Since they stopped allowing the use of PCB’s the capacitors now have a shelf life. Many times I see capacitor problems that will not allow the compressor or the fan to come on. Many times you can clearly see that the capacitor is bad because it is swollen or even blown apart with capacitor oil everywhere! Please see the pictures below.  Sometimes you need a special meter to test the microfarad (MFD) or UF rating. A capacitor tester. Please click here if you are interested in seeing a real good Supco Capacitor tester.

“MFD” and “UF” mean the same thing, “Microfarad”. Many times you will see an “UF” rating on the capacitor instead of “MFD.” Yes, these two abbreviations mean the same. Most of the time you can tell that a capacitor is bad because it will be swollen up. Please see the picture below for the comparison between a good and bad dual run capacitor. We call them dual because the capacitor helps run both the fan and the compressor. If you have any questions please email us anytime or comment below. Our email address is:

*Tip: When identifying the capacitor that you have, you might see the rating stated in “MFD” or “UF.” “MFD” and “UF” mean microfarad rating and mean the same. Below we have some pictures of bad round and oval capacitors. Sometimes capacitors are weak and need to be replaced. If you do not have a capacitor tester…I would suggest taking the capacitor to an appliance parts supplier and ask them if they would be kind enough to test the capacitor for you. Please click here if you are interested is seeing the over 80 different capacitors we sell. We would love to help you out and have your business!

Bad round dual capacitor on the left.
Bad oval capacitor on the left.

Below we have a real good informative video which shows how to troubleshoot and repair an air conditioner’s contactor and capacitor. Thanks to Kevin Miller for making this informative video!

Below we have a real good informative and educational video on “Dual Capacitor Checkout Procedure” made by Ron Walker. Thanks so much to Mr. Walker for making this informative video!

10 thoughts on “Problem: What to look for when a capacitor is bad? My unit will not do anything!

  1. I am getting a Status Code 41 on my t-stat which is, motor control has detected that the motor will not come up to speed within 30 seconds of being commanded to run or that the motor has been slowed to below 250 rpm for more than 10 seconds after coming up to speed. My capacitor is testing a bit low and not within the 6%. I was getting 4.2 out of 5uf. The other side of the dual capacitor was giving me the full 35uf. Could my capacitor be the cause of this?

    1. Hi Jonathan! I am sorry that I am not familiar with a system that sends the status codes through the thermostat. You must have a pretty high efficient system with lots of electronics. Sorry I have not been doing any service work since 2005. I would think, in what little I know that the code would be referring to the blower motor on your furnace or air handler. If the code is coming from the outdoor unit then yes, I would suggest replacing the dual run capacitor on the unit because 4.2 uf is a little low and it could affect the speed of the outdoor fan motor. I always heard that a capacitor should be within + or minus 10% and 4.2 is below 10% of 5uf. So I would recommend replacing the capacitor. Sorry that I can not give you much help. Steve

  2. Can u replace a 45+10 with a 45+5

    1. No sir, you can not replace a 45/10 with a 45/5 capacitor. The fan would not work properly and probably would not work at all. The voltage is important too. You can always go up in voltage, but never down. For example if you have a 45/10 mfd 440 volt capacitor you need to replace it with a 440 volt capacitor, not a 370 volt capacitor. If you have a 370 volt capacitor you can go up to a 440 volt capacitor. I hope you have a great day!

  3. At the very top when you said “Weak or bad capacitors can also cause blower motors to not run or to run slow”. Did you mean condenser fan instead of blower motor? Just curious, because my condenser fan is only running about 20% as fast as it should and the compressor is grinding as it fails to start. Tested the cap and it was ok, but I only have an analog meter.

    1. Hi Carl! Weak or bad capacitors can cause both blower motors and condenser fan motors to run slow. If your condenser fan motor is running slow it could be a weak capacitor or a condenser fan motor where the bearings are dragging and the motor is on its way out. You will need a capacitor tester to test capacitors. We have lots of air conditioning and heat pump troubleshooting questions with answers and Youtube videos on the following page: We also have an air conditioning and heat pump troubleshooting simplified page that I hope will help you find and fix the problem: I hope you have a great day! I hope you can easily find and fix the problem. Steve

  4. can you replace a 40/5 uf 440 vac capacitor with a 45/5uf 440 vac capacitor.

    1. Hi David!
      I would not recommend doing this because manufacturer’s recommend plus or minus 10% 10 percent of 40 of course would be 4 uf and you are asking about 5 uf difference so I would not recommend this. I would recommend the exact replacement because compressors are too expensive to take chances. Steve

  5. My unit has a single capacitor. My local HVAC parts dealer does not have the single, but has a Dual unit and they say I can use that. The one I need is a 43/370. The one they gave me is a 45/3/370. They say not to use the 3. I have the new capacitor and it is labeled C, FAN, Hxxx (hard to make out). First can I use this capacitor? and what wires go to what. Your description above for the Dual says the 3 is for the fan. This is why I am confused, I think my fan unit is bad. They say use the 45/370, but the capacitor has FAN on it but your website says that is the 3uf. Please clarify…..

    1. Hi Andrew! Did you watch the Youtube videos that are on our page: They should explain. Yes, you can use a dual capacitor to replace a single capacitor. The capacitor that you want replaced I think from your description is the 43 MFD/uf 370 volt single capacitor. Yes, your service people could use the 45/3 MFD/uf 370 volt capacitor as a replacement since the 45 is not 10% more than the 43 that you want. This capacitor is for the compressor only and you should only use the “Herm” and “Com” terminals on the capacitor. The “Herm” connection wire would go to the start terminal on the compressor and the C (com) wire would go to a power source usually from the contactor. You would not use the “Fan” terminal on the capacitor because as far as I understand the capacitor is not being used for the fan only for the compressor. You probably have another capacitor the fan is using. I hope you can easily fix your air conditioner or heat pump. Steve

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