Capacitors & Contactors: The Small Parts That Stop Cooling

Q: My air conditioner suddenly stopped cooling, won’t turn on, or makes a humming sound. Someone told me it might be a capacitor or contactor. What are these parts, and how do they affect my AC?

A: Capacitors and contactors are small electrical parts that play a big role in how your air conditioner runs. When either one fails, your AC may stop cooling, fail to turn on, or shut off unexpectedly. These are some of the most common AC repair issues our friendly technicians in yellow polos see in Florida—especially during the long cooling season.

Here’s what Citrus County homeowners should know about these components—and why these repairs should always be handled by a trained HVAC professional.

See more answers in the Air Conditioning Help Center.

1. What a Capacitor Does

Your AC has one or more capacitors that store and release electrical energy. They help motors start and run, including:

• Compressor motor
• Fan motor (outdoor unit)
• Blower motor (indoor unit)

Think of a capacitor like a “battery boost” that gives the motors enough power to start and stay running.

Common symptoms of a bad capacitor:

• AC makes a humming noise but doesn’t start
• Outdoor fan isn’t spinning
• Warm air blowing from vents
• AC starts and stops frequently
• Breaker trips when the system tries to start

Capacitors wear out naturally over time, especially in Florida heat. A failed capacitor can stop your AC from cooling and, in some cases, prevent it from starting at all.

2. What a Contactor Does

The contactor is a switch that controls power to your outdoor AC unit. When your thermostat calls for cooling, the contactor closes and sends electricity to the compressor and fan motor.

If the contactor fails, your AC may:

• Not turn on at all
• Turn on but not cool properly
• Shut off randomly
• Make a clicking or buzzing sound

Contactors can fail from age, insects (yes, this happens often in Florida), heat, or electrical wear.

3. Warning: These Are Not DIY Parts

Capacitors and contactors store and control high voltage electricity. Even when the system is off, capacitors can hold a dangerous charge that can cause severe injury if mishandled.

Beacon strongly recommends against trying to replace these parts yourself.

Why?

• Incorrect capacitor sizing can damage the compressor.
• Mishandled wiring can create shorts or fire hazards.
• A wrong diagnosis can lead to more expensive repairs later.

These repairs should always be done by a licensed HVAC professional who has the proper meters and discharge tools.

4. How Beacon Diagnoses These Issues

When you call Beacon Services & Appliances, Beacon’s friendly technicians in the yellow polo shirt perform a full electrical diagnostic, including:

• Testing the capacitance (µF) levels
• Checking for heat damage or swelling
• Inspecting contactor surfaces for pitting or carbon buildup
• Testing voltage and amperage during system startup
• Inspecting wiring connections and relays

We use high-quality replacement parts that meet or exceed manufacturer specifications.

5. Can a Failing Capacitor or Contactor Cause Other Damage?

Yes—if left unaddressed, these parts can contribute to damage in more expensive components:

• Compressor strain from hard starts and low starting torque
• Fan motor wear from repeated failed starts
• Repeated breaker trips that stress electrical components

If your system hesitates to start or makes unusual noises, it’s best to have it inspected quickly.

6. When to Call Beacon

If your air conditioner:

• Won’t turn on
• Turns on but doesn’t cool
• Hums when trying to start
• Trips the breaker
• Starts and stops repeatedly

…a capacitor or contactor could be the issue.

Beacon’s friendly technicians in the yellow polo shirt can diagnose the problem, replace the faulty part, and fully test your system to ensure it’s running safely and efficiently.

Explore more helpful AC guides:

• Why Is My Air Conditioner Not Cooling?
• Why Is My Air Conditioner Freezing Up?
• How to Lower Your Electric Bill Without Sacrificing Comfort
• What Is SEER2 and Why Does It Matter?