How to Replace an AC Capacitor

How to Replace an AC Capacitor

Step-by-step guide for replacing the capacitor in your condenser (outdoor unit) or air handler (indoor unit). Save $300+ by doing it yourself.

⚠ Important Safety Notice & Disclaimer

Working with electrical components carries inherent risk of shock, injury, or death. Capacitors store electrical charge even after power is disconnected and can deliver a dangerous shock. Always disconnect power at the breaker AND the outdoor disconnect before beginning any work. Verify power is off with a multimeter before touching any wires.

This guide is provided for educational purposes only. Open To Public HVAC Parts makes no warranties regarding the accuracy or completeness of this information. By following this guide, you acknowledge that you do so at your own risk. Open To Public HVAC Parts, its owners, and employees are not liable for any injury, damage, or loss resulting from the use of this information. If you are not comfortable working with electrical components, hire a licensed HVAC technician.

What Is an AC Capacitor?

A capacitor is a small metal cylinder that stores and releases electrical energy to start and run the motors in your AC system. Think of it like a battery boost — it gives your compressor and fan motor the extra kick they need to start up, then keeps them running efficiently.

Closeup of a dual run capacitor showing terminals and connections
A dual run capacitor — the cylinder that stores energy to start and run your AC motors.

There are two types you’ll find in residential HVAC systems:

Two different sized dual run capacitors side by side showing size varies by tonnage
Top view of dual run capacitor showing three terminals HERM FAN and COM

Left: Two dual run capacitors — different sizes for different tonnage systems. Right: Top view showing the three terminals (HERM, FAN, C).

Dual Run Capacitor

The most common type in residential condensers. One capacitor runs both the compressor and the fan motor. It has three terminals on top labeled HERM (compressor/hermetic), FAN (condenser fan motor), and C or COM (common). The rating is written as two numbers like “45+5 MFD” — the first number (45) is the compressor side, the second (5) is the fan motor side.

Top view of dual run capacitor showing three terminals HERM FAN and COM
Top view of a dual run capacitor — three terminals labeled HERM, FAN, and COM.
Wiring connections on a dual run capacitor — HERM, FAN, and C terminals
Wiring connected to a dual run capacitor — each terminal feeds a different motor circuit.

Single Run Capacitor

Has only two terminals. Used for single motors — you might find one inside your air handler running the blower motor, or occasionally in a condenser that uses separate capacitors. The rating is a single number like “10 MFD.”

Close-up of capacitor label showing microfarad MFD rating and voltage rating

The label tells you everything you need: the MFD (microfarad) rating and the voltage rating (370V or 440V). Match these numbers when buying a replacement.

Signs Your Capacitor Might Be Bad

Capacitors are one of the most common parts to fail in an AC system, especially in Dallas-area heat where they’re working hard from April through October. Here’s what to watch for:

AC Won’t Start / Hums But Doesn’t Turn On

The outdoor unit hums or clicks when the thermostat calls for cooling, but the compressor and fan don’t kick on. The capacitor doesn’t have enough juice left to start the motors.

Fan Spins Slowly or Struggles to Start

The condenser fan barely turns, wobbles, or needs a push to get going. That’s a weak capacitor losing its ability to start the motor.

AC Shuts Off Randomly

The system runs for a while then cuts out, especially on the hottest days. A weak capacitor can’t sustain the motor under heavy load.

Swollen or Leaking Capacitor

If the top of the capacitor is bulging (it should be flat) or you see oily residue, it’s toast. This is the easiest visual confirmation — a healthy capacitor has a perfectly flat top.

Pro Tip: Not sure if it’s the capacitor? Bring it to Open To Public HVAC Parts — we’ll test it on our bench for free in about 30 seconds. No guessing, no appointment needed.

How to Test Your Capacitor

Before replacing, you can test your capacitor with a multimeter that reads microfarads (MFD or uF). Most digital multimeters have this setting.

Testing an AC capacitor with a Fieldpiece multimeter at Open To Public HVAC Parts
Testing a capacitor with a multimeter — checking the microfarad rating to see if it’s within spec.
Fieldpiece SC260 multimeter being used to test AC capacitor microfarad reading
Meter probes connected to Titan HD dual run capacitor for MFD testing

Left: Set your meter to the microfarad (MFD) setting. Right: Touch the probes to two terminals to read the capacitance.

1

Disconnect Power and Discharge the Capacitor

Turn off the breaker and the outdoor disconnect. Use an insulated screwdriver to short across the terminals to discharge any stored energy. This is critical — a charged capacitor can shock you.

2

Remove the Wires

Pull the wire connectors off each terminal. Take a photo first so you remember which wire goes where.

3

Set Your Meter to Microfarads

Turn the dial to the MFD/capacitance setting. If your meter has multiple ranges, choose the one that covers the number on your capacitor’s label.

4

Read the Value

For a dual run capacitor, test both sides separately. Touch probes to HERM + C to read the compressor side, then FAN + C for the fan motor side. For a single run capacitor, just touch the two terminals.

Reading capacitor test results on a digital multimeter display
Reading the capacitor test result on the multimeter display.
Measuring capacitor microfarad rating with a Fieldpiece SC260 clamp meter
Measuring the microfarad rating — compare this number to the rating printed on the capacitor.
5

Compare to the Rating

The reading should be within 6% of the rated value. A 45 MFD cap should read between 42.3 and 47.7. Anything outside that range — the capacitor needs to be replaced.

Don’t have a meter? No problem. Bring the capacitor to our counter at Open To Public HVAC Parts and we’ll test it for free. We have professional bench-testing equipment that gives an instant reading. (214) 340-9421

Watch: AC Capacitor Test — How To Check Microfarads With a Multimeter (Open To Public HVAC School)

What You’ll Need

Tools

  • 5/16″ or 1/4″ nut driver (to remove the access panel screws)
  • Insulated screwdriver (to discharge the capacitor)
  • Multimeter (to verify power is off and test the old cap)
  • Phone or camera (to photograph the wiring before you disconnect anything)
  • Needle-nose pliers (helpful for pulling wire connectors off tight terminals)

Parts

  • Replacement capacitor — match the MFD rating and voltage (370V or 440V) exactly
  • Universal mounting strap (if the old bracket doesn’t fit the new cap)
On the voltage rating: You can always go up in voltage but never down. A 440V capacitor works fine as a 370V replacement, but a 370V cap cannot replace a 440V. When in doubt, go with 440V.

Step-by-Step Replacement

The process is similar whether you’re working on the outdoor condenser or the indoor air handler. Select your unit below:

Overview of AC capacitor replacement process in a condenser unit
Overview of the condenser unit with the access panel removed, ready for capacitor replacement.
Condenser (Outdoor Unit)
Air Handler (Indoor Unit)

1

Turn Off the Power

Turn off the AC at the thermostat. Then go to your breaker panel and flip off the breaker for the outdoor unit. Finally, pull the disconnect at the outdoor disconnect box. Do all three.

Inside of a condenser unit with power off - safety check before capacitor replacement

Always verify power is completely off before touching anything inside the unit.

Do NOT skip this step. A capacitor can hold a charge even after the power is turned off. You must discharge it (Step 3) before touching any wires.
2

Remove the Access Panel

The capacitor is behind the electrical access panel on the side of the condenser. Remove the screws with your 5/16″ or 1/4″ nut driver and set the panel aside.

3

Discharge the Capacitor

Before touching any wires, use an insulated screwdriver to short across the terminals. For a dual run cap, short HERM to C and FAN to C. You might see a small spark — that’s normal.

4

Photograph the Wiring

This is the most important step people skip. Take a clear photo of the wires connected to the capacitor before you disconnect anything.

Closeup of capacitor wire connections during replacement
Take a clear photo of all wire connections before disconnecting anything.

Capacitor wiring inside condenser unit showing colored wire connections

Note the wire colors and which terminal each one connects to. Your wiring may look different.

5

Disconnect the Wires

Pull each wire connector straight off the terminal. Don’t yank sideways — the terminals can bend or break. Use needle-nose pliers if a connector is stuck.

6

Remove the Old Capacitor

The capacitor is held in place by a metal bracket or strap. Unscrew or unclip it and pull the old cap out. Note the MFD rating and voltage on the label.

7

Install the New Capacitor

Mount the new capacitor in the bracket. If it’s a different physical size, a universal mounting strap works great.

Installing a new dual run capacitor in an air conditioning condenser
Installing the new capacitor — mount it securely before reconnecting wires.

Condenser access panel being replaced after successful capacitor installation

The new capacitor goes in the same spot. Reconnect the wires to the matching terminals using your reference photo.

8

Reconnect the Wires

Using your photo from Step 4, push each wire connector back onto the correct terminal. Make sure each connection is snug.

Testing the AC system after installing new capacitor

Standard Wiring Reference

Dual Run Capacitor Wiring DiagramHow the wires connect — Condenser UnitHFCHERMFANCOMCONTACTORCOMPRESSORFAN MOTORYELLOWBROWNREDWire Colors May Vary:HERM = Yellow | FAN = Brown | COM = Red (from contactor) | Always match wires to the old cap

Wire colors vary by manufacturer. The diagram shows typical colors, but your system may be different. Always go by your photo. If you’re unsure, bring the old cap and a photo of the wiring to Open To Public HVAC Parts — we’ll help you figure it out.
9

Replace the Panel and Restore Power

Screw the access panel back on. Reinsert the disconnect. Turn on the breaker. Set your thermostat to cool, a couple degrees below room temperature, and listen. You should hear the compressor and fan kick on within a minute.

The capacitor in your air handler (the indoor unit, usually in a closet, attic, or garage) runs the blower motor. It’s typically a single run capacitor with only two terminals.

1

Turn Off the Power

Turn off the AC at the thermostat, then flip the breaker for the indoor unit. Some air handlers also have a service switch — turn that off too. Verify with a multimeter that there’s no voltage present.

2

Open the Blower Compartment

Remove the access panel or door on the air handler. The capacitor is usually mounted on a bracket near the blower motor or on the side wall of the compartment.

3

Discharge and Photograph

Same as the condenser — discharge the capacitor by shorting across its two terminals with an insulated screwdriver, then photograph the wiring before disconnecting anything.

4

Swap the Capacitor

Remove the old single run cap, note the MFD and voltage rating, and install the new one. Reconnect the two wires — on a single run capacitor, it doesn’t matter which wire goes to which terminal (they’re interchangeable).

Single run caps are simpler: Unlike dual run caps where the terminals are labeled HERM/FAN/C, a single run capacitor’s two terminals are interchangeable. Either wire can go on either terminal.
5

Close Up and Test

Replace the access panel, restore power, and set the thermostat to call for heating or cooling. Listen for the blower motor to start. It should spin up smoothly without hesitation.

ECM / Variable Speed Blowers: If your air handler has an ECM (electronically commutated motor) or variable-speed blower, it does NOT use a traditional capacitor. These motors have built-in electronics. If you don’t see a capacitor near the blower motor, it’s probably an ECM system.

DIY vs. Calling a Pro

A capacitor replacement is one of the most straightforward AC repairs you can do yourself. Here’s what you’ll save:

Expense DIY (Open To Public) HVAC Service Call
Capacitor $15 – $35 $50 – $150 (marked up)
Service / Diagnostic Fee $0 $89 – $150
Labor $0 (15 minutes of your time) $75 – $200
Total $15 – $35 $214 – $500

That’s a savings of $180 to $465 in your pocket — for about 15 minutes of work.

Watch the Full Video Guide

Want to see the entire process from start to finish? Watch our step-by-step video tutorial:

AC Capacitor Replacement — How To Replace Your Capacitor Step by Step (Open To Public HVAC School, 13K+ views)

Need a Capacitor? We’ve Got It.

Open To Public HVAC Parts carries every common capacitor size in stock — dual run, single run, round, and oval. We stock American-made Titan HD capacitors and import options. Bring in your old one and we’ll match it on the spot. We’ll even test it for free to make sure that’s actually the problem before you buy.

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