How To Check The Capacitor On An AC Unit: Essential Steps, Signs, And Safety Tips For Homeowners

When your air conditioning system starts malfunctioning, a faulty capacitor is a common culprit. The capacitor is vital for starting and running the AC’s compressor and fan motor. Knowing how to safely check the capacitor on your AC unit can save you money and help decide if you need professional repairs. Below is a quick reference table to assist you before diving into the step-by-step process.

Indicator	Likely Problem	Test Method	DIY Friendly?
Humming Sound, No Start	Failed Start Capacitor	Multimeter Capacitance Test	Yes, With Caution
Fan Spins Slowly or Won’t Start	Failed Run Capacitor	Physical Inspection & Capacitance Test	Yes, With Precautions
Bulging/Leaking Capacitor	Capacitor Is Bad	Visual Inspection	Yes, Simple

Understanding The Role Of The AC Capacitor

The capacitor in your AC unit stores and releases electrical energy to give the compressor and fan motor the boost they need to start and run. Most modern central AC units utilize two types: the start capacitor and the run capacitor. The start capacitor provides the initial jolt for startup, while the run capacitor helps maintain a steady current during operation. Without a functioning capacitor, the AC cannot run efficiently—or may not run at all.

Warning: Safety First When Checking AC Capacitors

Capacitors store a significant electrical charge, even after the power is turned off. A shock can be dangerous. Always turn off the power at the breaker and use a non-contact voltage tester to confirm. Discharge the capacitor using an insulated screwdriver with a resistor, wearing insulated gloves and eye protection. If unsure, consult a licensed HVAC technician instead of proceeding.

Symptoms Of A Bad AC Capacitor

• AC Won’t Turn On: The compressor or fan fails to start, even when the thermostat is calling for cooling.
• Humming Noise: The outdoor unit hums but doesn’t turn on.
• Fan Won’t Spin: You may manually start the fan blades with a stick, and it works, but stops when off.
• Intermittent Cooling: The unit cools erratically or stops mid-cycle.
• Bulging or Leaking Capacitor: The top of the capacitor is swollen, leaking oil, or shows scorch marks.

If you notice any of these issues, it’s time to check the capacitor.

Tools And Equipment Needed

• Multimeter (with capacitance setting preferred)
• Insulated Screwdriver (for discharging capacitor)
• Non-Contact Voltage Tester
• Insulated Gloves & Safety Glasses
• Camera or Smartphone (to record wire connections)
• Replacement Capacitor (if needed, match ratings exactly)

Step-By-Step Guide: How To Check Your AC Unit’s Capacitor

1. Turn Off All Power To The AC Unit

Locate your home’s circuit breaker panel. Switch off the breaker labeled for your air conditioner. There may also be a separate disconnect near the outdoor unit—open it and remove the fuse or switch to “OFF” for total safety.

2. Access The Capacitor

Remove the screws securing the access panel on the AC unit’s outdoor condenser. Carefully lift away the panel to reveal the electrical components. The capacitor is usually a silvery cylinder or oval canister attached with wires and a mounting strap, often labeled “start” and “run.”

3. Discharge The Capacitor

With all power off, use a non-contact voltage tester to confirm no live electricity. To discharge, hold an insulated screwdriver (with a resistor or using pure caution) by the handle and touch both terminals simultaneously. Some capacitors will spark slightly. Once discharged, it’s safe to proceed.

4. Document The Wire Connections

Take a clear photo of the capacitor’s wiring before disconnecting anything. AC capacitors typically have three terminals: “C” (common), “FAN,” and “HERM” (hermetic compressor). Having a picture makes reinstallation much easier.

5. Remove The Capacitor

Use needle-nose pliers with insulated handles to gently pull the spade connectors off the capacitor terminals. Remove the mounting strap or bracket and carefully take out the capacitor.

6. Perform A Visual Inspection

Look for signs of bulging, leaks, or corrosion. A good capacitor should have a flat top and no visible fluids. Bulging, splits, or oily residue indicate it should be replaced without further testing.

7. Test The Capacitor With A Multimeter

• Set your digital multimeter to the capacitance (μF) setting.
• Touch the multimeter probes to the “C” (common) and “FAN” terminals, then “C” and “HERM.”
• Compare the reading against the capacitance range printed on the capacitor’s label (example: 35+5 μF ±6%).
• If the reading is more than 10% lower than the rated value (example: below 31.5 μF for a 35 μF capacitor), the capacitor should be replaced.

Interpreting Capacitor Test Results

Capacitor Condition	Capacitance Reading	Action Needed
Good	Within ±6-10% Of Rating	Reinstall
Marginal/Weak	10-20% Below Rating	Replace Soon
Bad	Over 20% Below Or No Reading	Replace Immediately

Choosing The Right Replacement Capacitor

It’s crucial to match the microfarad rating (μF) and voltage exactly as specified on your original capacitor. Using a wrong rating can damage your AC unit or reduce efficiency. If your unit uses a dual-run capacitor, the form will list two μF numbers (such as 45/5). The voltage (VAC) should be equal or higher but never lower than the original.

How Long Do AC Capacitors Last?

Most AC capacitors last 8-15 years under normal conditions. Exposure to high heat, frequent starts, or voltage irregularities can shorten that lifespan. Replacing the capacitor proactively when weak can prevent further damage to the compressor or fan motor.

Common Causes Of AC Capacitor Failure

• Age And Wear: Over time, internal chemicals break down, reducing performance.
• Overheating: Heat exposure from sun, high amp draw, or constant running.
• Voltage Surges: Lightning strikes, utility swings, or nearby power outages.
• Poor Quality Or Improper Replacement: Low-quality parts or mismatched ratings fail quickly.

Can A Bad Capacitor Damage The AC Unit?

Yes—a failing capacitor can cause hard starts, overheating, and may strain or burn out the compressor or fan motor. This can turn a $20-50 component repair into a multi-thousand-dollar replacement if ignored. That’s why timely detection and replacement are critical.

DIY Or Call A Professional?

If you are comfortable working with electrical components, have the proper tools, and follow safety guidelines, replacing a capacitor can be a simple DIY project. However, hiring a certified HVAC technician is best for most homeowners. They can test for additional problems, such as motor burnout or bad relays, that might also cause similar symptoms.

Frequently Asked Questions About AC Capacitors

Is It Dangerous To Test A Capacitor On My Own?

Capacitors can hold a dangerous charge after power-off. Only proceed if fully confident and prepared—otherwise, contact a professional.

Can I Use A Higher Voltage Rating Capacitor?

The microfarad (μF) rating must match exactly, but the voltage rating (VAC) can be equal or higher than the original.

How Can I Tell If My AC Trouble Is Not The Capacitor?

Capacitor-related issues often cause humming, slow or failing fans or compressors, and visible swelling. If your unit trips breakers, leaks refrigerant, or never powers on at all, other problems may exist.

Pro Tips For AC Maintenance To Prevent Capacitor Failure

• Keep Outdoor Units Cool: Plant shade trees or install covers (without blocking airflow).
• Have Annual Professional Tune-Ups: Early detection can save your AC.
• Change Air Filters Regularly: Good airflow reduces system strain.
• Check For Rodents Or Insects: They can damage capacitors and wiring.

Summary Table: Steps To Check An AC Capacitor

Step	Action	What To Look For	Next Move
Turn Off Power	Shut off breakers and check voltage	Zero voltage	Open unit and proceed
Discharge Capacitor	Use insulated tool	No spark means safe to proceed	Document wiring
Visual Inspection	Check for bulge, leak	Damaged? Replace without testing	Test with multimeter if good
Measure Capacitance	Multimeter on “μF” between terminals	Within 10%? Good	If low, replace with correct rating

Key Takeaways For Homeowners

• Always prioritize safety before opening an AC unit—electric shock risk remains, even unplugged.
• Visible bulging or leakage means immediate replacement is necessary.
• Match replacement capacitors’ μF and voltage ratings exactly to manufacturer specs.
• When in doubt, engage a licensed HVAC technician for repairs and diagnostics.

If you’re experiencing AC issues and suspect a blown capacitor, follow these steps to check the part before replacing it. A functioning capacitor keeps your air conditioner running efficiently and protects expensive components from damage.