If your air conditioner is humming but won’t start, or the fan is running but there’s no cool air, a failed capacitor is one of the most common causes. The good news is that AC capacitor replacement is one of the more straightforward HVAC repairs — but you need to take safety seriously because capacitors store a dangerous electrical charge.
What Does an AC Capacitor Do?
Capacitors store and release electrical energy to start and run the motors in your AC system — specifically the compressor and the condenser fan. There are two types: start capacitors (provide a boost to get the motor going) and run capacitors (keep the motor running efficiently). A dual-run capacitor handles both jobs in one unit, which is what most modern AC systems use.
Signs of a Bad Capacitor
- AC unit hums but doesn’t start
- The fan spins slowly or not at all
- The system shuts off shortly after starting
- Higher electric bills with less cooling
- A bulging or leaking capacitor (visible when you open the unit)
Safety Warning
Before you touch anything inside your AC unit, you MUST discharge the capacitor. Even with the power off, a capacitor can hold hundreds of volts. Touching a charged capacitor can cause a serious — potentially fatal — electric shock. Use an insulated screwdriver to discharge it by bridging the terminals before handling it.
Tools and Parts You’ll Need
- Replacement capacitor (match the microfarad (µF) and voltage rating exactly)
- Insulated screwdrivers (flathead and Phillips)
- Needle-nose pliers
- Multimeter (optional but helpful for confirming diagnosis)
- Phone or camera for photos before disconnecting wires
Step 1: Shut Off Power Completely
Turn off the AC at the thermostat. Then go to your electrical panel and switch off the breaker for the AC condenser. There should also be a disconnect box near the outdoor unit — pull the disconnect or flip the switch inside it. Do not skip this step.
Step 2: Access the Capacitor
Remove the access panel on the side of your outdoor condenser unit — usually held by one or two screws. The capacitor is the cylindrical component, often silver or black, mounted near the top of the panel. It will have two or three sets of terminals labeled HERM (hermetic compressor), FAN, and COM (common).
Step 3: Discharge the Capacitor
Using an insulated flathead screwdriver, touch the blade across the HERM and COM terminals, then across the FAN and COM terminals. You may hear a small pop or see a small spark — that’s normal and means it’s discharging. Do this before touching any wires.
Step 4: Photograph and Remove the Old Capacitor
Take a clear photo of the wiring before disconnecting anything. Use needle-nose pliers to pull the wire terminals off (don’t yank the wires themselves). Note the µF rating and voltage stamped on the side of the old capacitor so you buy the correct replacement.
Step 5: Install the New Capacitor
Mount the new capacitor in the same bracket. Reconnect the wires exactly as they were in your photo — HERM, FAN, and COM go back to the same terminals. Push the wire connectors firmly onto the terminals until they click or seat snugly.
Step 6: Restore Power and Test
Replace the access panel, restore power at the disconnect box and breaker, and turn the AC on at the thermostat. The unit should start up normally within a few seconds. Let it run for 10–15 minutes and check that cool air is coming from your vents.
When to Call an HVAC Tech Instead
If you replace the capacitor and the AC still doesn’t start, the problem may be a failed compressor, a bad contactor, or a refrigerant issue — all of which require a licensed HVAC technician. If you’re not comfortable working around electrical components at all, there’s no shame in calling a pro for this one.