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 Is an AC Capacitor and What Does It 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 running) and run capacitors (keep the motor running efficiently). Most modern systems use a dual-run capacitor that handles both functions in one component.
How to Tell If Your Capacitor Is Bad
- AC unit hums but won’t turn on
- The fan is spinning slowly or not at all
- System kicks on but shuts off quickly
- Rising energy bills with reduced cooling output
- Visible bulging or oil leaking from the capacitor body
Important Safety Warning
Before touching anything inside your AC unit, you must discharge the capacitor. Even with the power off, a capacitor can hold hundreds of volts of charge. Accidentally touching a charged capacitor can cause a severe — potentially fatal — electric shock. Always use an insulated screwdriver to discharge it by bridging the terminals before you handle it.
What You’ll Need
- Replacement capacitor (must match the microfarad (µF) and voltage rating exactly)
- Insulated screwdrivers (flathead and Phillips)
- Needle-nose pliers
- Multimeter (optional, but good for confirming the diagnosis)
- Camera or phone to photograph the wiring before disconnecting
Step 1: Cut the Power
Switch off the AC from your thermostat. Then head to your electrical panel and turn off the breaker for the outdoor condenser. There should also be a disconnect box mounted near the outdoor unit — open it and pull the disconnect or flip the switch. Always cut power at both points before proceeding.
Step 2: Open the Access Panel
Remove the access panel on the side of your outdoor condenser unit — it’s typically held in place by one or two screws. Inside, you’ll find the capacitor: a cylindrical component, usually silver or black, mounted near the top. It has two or three terminal sets labeled HERM (hermetic compressor), FAN, and COM (common).
Step 3: Discharge the Capacitor
Take an insulated flathead screwdriver and bridge the blade across the HERM and COM terminals, then across the FAN and COM terminals. A small pop or spark is normal — it means the stored charge is releasing safely. Never skip this step.
Step 4: Document and Remove the Old Capacitor
Snap a clear photo of the wiring setup before you touch anything. Use needle-nose pliers to remove the wire terminals (pull from the connector, not the wire). Check the µF and voltage ratings printed on the old capacitor so you can get an exact replacement.
Step 5: Install the Replacement Capacitor
Seat the new capacitor in the same mounting bracket. Reconnect each wire to the correct terminal using your photo as a reference — HERM, FAN, and COM terminals should each get the same wire they had before. Press the connectors on firmly until they seat securely.
Step 6: Power On and Test the System
Reattach the access panel, restore power at the disconnect and breaker, then turn on the AC at the thermostat. The unit should start normally within a few seconds. Let it run for 10–15 minutes and confirm that cool air is flowing from your registers.
Still Not Working? It’s Time to Call a Pro
If the AC still doesn’t start after replacing the capacitor, you may be dealing with a failed compressor, a burned-out contactor, or a refrigerant problem — all jobs for a licensed HVAC technician. If electrical components make you uncomfortable, it’s perfectly reasonable to call a pro for this repair from the start.