How to Replace a Central Air Conditioning Capacitor (with diagnosis)9 min read

How to Replace a Central Air Conditioning Capacitor (with diagnosis)9 min read

A Simple Step-by-Step Guide to Replace an AC Capacitor

Is it hot and have a malfunctioning or not working air conditioning unit? Chances are your AC unit has an issue with the electrical. In fact, the majority of the AC unit problem is related to electrical so you are safe to assume so. And that starts with replacing your AC capacitor.

Now I had the same happen to me last month when it was 90 degrees outside.  I immediately ended up calling a technician to come over just to figure out that someone played with the breaker.  I know, I felt so stupid not to catch that.

Anyways the HVAC tech tried to upsell me on charging the unit and changing the capacitor. He basically scared me saying that the capacitor is rusted and it might be bad or can bust anytime.

I did change as he suggested and ended up paying upwards of $300.  Mostly because I don’t have one handy and online delivery will take a good few days and I am not sure big box carries them.

Buy one and keep it handy, so you can change when it’s near the end of life.

Nearing the end of life on your HVAC? Not sure to replace or repair? Save on a new Heating and AC System!

Do you know that the best of the best capacitor is going to be less than $20?  You can buy it online. Any aftermarket one would work just fine – just remember to buy the rating (MFD and Voltage) to match your old one.

And to replace a central air conditioning capacitor it is just a piece of cake. It’s much easier than you can think. If you can swap a switch or an outlet, a capacitor is no more difficult.

What is the Role of a Capacitor in an AC Unit?

The capacitor is a commonly found electrical (and electronic) component in a variety of gadgets and equipment you see.  In a nutshell, the capacitor regulates or provides enough juice (power) needed for certain parts of the appliance to function properly.

If you notice the blinking of turning lights in a car is achieved by the use of a capacitor. So you get the point. 

In a central air conditioning unit though, a capacitor is required to provide a level of power needed for compressor and motors to kick start.  Some of the components have a more than usual power draw at the start of the operation.  

So the capacitor provides the needed power for a smooth start by providing the level of power needed. 

How to Identify a Failing Capacitor (Symptoms)

Every AC component has a life. The capacitor is no exception.  In essence, they lose the ability to hold the much-needed power.  But there are times when power surges happen blowing away the capacitor. 

In any case, you will find the following symptoms helpful in identifying a failing capacitor. 

  • Use a multimeter and check for capacitance – it should be within the tolerance limit of the suggested MFD rating.
  • Click sound every time when the central AC unit comes on
  • AC motor makes a humming sound, trying to run but couldn’t get itself to start or run
  • The air condition unit has trouble starting but keeps turning off.
  • Visual signs – The top and the bottom of the capacitor will become puffy or bulged.
Identify failing capacitor by looking for bulge or swelling at the top or bottom

In any of these situations, it’s a telltale that the capacitor is going bad or has already failed.  If the condition continues it will burn the motor and strain other parts. 

If you don’t replace it immediately it will subsequently fail other components causing more trouble.

DIY AC Unit Capacitor Replacement

This is most definitely a DIY project.  I wouldn’t even call it a project, it’s just a simple task and extremely DIY friendly.

But like with any electrical work, you must exercise caution and turn the breaker off. 

Here is another important thing to remember. 

The Capacitor is used to store energy, much like a battery. So if you touch it, even with breakers turned off, you may shock yourself. 

Don’t touch it with your bare hands. You can leave a few minutes after turning off, so the power is dissipated.   

Step by step guide to help you replace the capacitor

how to replace a new ac capacitor and wire them correctly

Step 1:  Buy a Correctly Rated Capacitor

Research the make and model and find the right capacitor requirement for your unit.  If you are unsure, follow steps 2 and 3 and take a picture of the capacitor. 

The Capacitor is usually an aluminum can like the component with three prongs on the top and mounted to the side when you open. It’s always the first component that you can see when you open the panel.

The capacitor will have technical information sticker  with the needed information — capacitance, tolerance, and the load voltage

Step 2:  Turn the Breaker Off and Cut the Power Supply

When you are working on the outside AC unit, you will find a breaker box or a fuse box right next to the AC unit (on the wall).  Turn the power off.

Warning: Most AC units get a 220V supply. That’s more than enough to kill someone, so use caution and turn it off. Do not touch anything inside the breaker box.

You may also try to turn the power off from the main circuit breaker panel if you can find the one that goes to the AC unit. 

Also, it is not a bad idea to turn the thermostat OFF during this time.  These are all steps for extra caution, but an experienced contractor will just turn the outside breaker off. 

Step 3:  Remove the Access Panel  

One of the sides of the unit should have nut screws.  You have to remove it and slightly wiggle the access panel and lift it up so it comes off totally.

It’s usually designed with 2 mounting screws at the top and two hooks at the bottom to secure all four corners. Sometimes all four corners will have screws.  Store them safely and don’t keep it on the grass, you may have a hard time finding it. 

Access Panel removal turning breaker off and replacing AC capacitor

Step 4: Discharge the Capacitor and Undo the Mounting Straps

I’m repeating the warning here…

Capacitors have one use – to store more power needed to provide the extra power needed for motor and compressor to kick start. 

But How to discharge AC Capacitor?

So you must exercise caution and not touch.  Instead, try to put the screw drive across two of the metal terminals.  This will short circuit and drain the power.

Also, give yourselves a few minutes after turning the power off so any power is dissipated. 

The capacitor is secured to the side of the unit by a mounting strap.  There should be a screw that you should loosen so you can remove the capacitor.

Step 5: Mark the Wires and Disconnect

AC Capacitor with 3 prong terminals for herm fan and common including compressor and fan rating minimum voltage and tolerance

This is a very important step. 

Check the wires – they should all be color-coded. Now you should notice three wires connected to the capacitor. 

In your old capacitor, there should be markings on the top.  Make a note of the letter marked for each of the terminal and the appropriate wire (color) going into it.

You may also have a wiring diagram (on some of the models) on the inside of the panel. If so, cross-reference the wiring and understand that well.

There are two ways to identify which is which – Markings and the number of prongs on each of the terminals 

  1. Four Prongs – “C”  = Common ( no compressor )
  2. Three Prongs – “HERM” = Compressor
  3. One (sometimes two) Prongs – “FAN” = Fan

Step 6: Replace a New AC Capacitor and Connect the Right Wires

Verify the new capacitor label to see you got the correct rated capacitor.  

Find the marking on the three terminals and reconnect the three wires.  Now put the capacitor back in the same position where you removed the old one from.

Secure the mounting straps and make sure it’s tight and not moving. 

Step 7: Turn the Power Breakers On 

Turn all the breakers on – Remember to turn the ones you turned off at the start of this exercise.  Go inside and set the thermostat at a nice cool temperature and turn the cool on.

Step 8: Test Run the Central AC

It should on and function properly.  You should check to see if you are getting cold air at the registers.  

However, if you don’t see the compressor or fan turn on you have one of two problems.

Your diagnosis of the problem is wrong and it is not a capacitor issue.  Which means you need to troubleshoot and fix something else. 

If you are right though, then you have wired the capacitor leads in the wrong way. Verify the wires and try to redo the above steps and you should get it to work this time.


This might look like a long article and confusing, but it’s not.  It is very easy.  

You are simply turning off the power, draining the stored power in the capacitor, removing the old and replacing it with a new airconditioning capacitor and reconnecting the correct wires.

It’s not hard. You can and should try. However, if you are scared of electric components or not sure how to do it, just call a technician.

But I just spent close to $400 for something this easy, is not cool. I am saving you time and money with this guide.