How to replace an AC run capacitor
- Takes about 15 minutes
- Beginner-friendly
- 4 tools needed
- Updated
- Reviewed by a licensed tech
If your condenser hums but the fan will not spin, a dead run capacitor is the first thing to check and the cheapest thing to fix. Fifteen minutes, one nut driver — and one safety step you genuinely cannot skip.
Safety — critical
Pull the disconnect at the condenser AND kill the breaker, then discharge the capacitor across a 20 kΩ / 2 W resistor before you touch a terminal. A run capacitor stores a lethal charge with the power off and holds it for hours — a screwdriver across the terminals will discharge it, damage it, and put an arc flash in your face. Use the resistor.
What you'll need
-
¼ in nut driver
For the capacitor strap and the service panel.
-
20 kΩ / 2 W resistor with insulated leads
The discharge tool. Read the safety note above — this is not the step to improvise.
-
Multimeter with a capacitance (µF) range
How you confirm the diagnosis instead of guessing at it.
-
Phone camera
Photograph the terminals before a single spade comes off.
The fix, step by step
-
Confirm it's the capacitor before you buy one
The classic tell: the compressor hums but the top fan does not turn, and a push on the fan blade with a stick starts it spinning. That is a capacitor that has lost its microfarads. A bulged or leaking top confirms it visually — but a flat-topped can can be just as dead, so do not clear it on looks alone.
-
Kill power at the disconnect AND the breaker
The disconnect is the pull-out box on the wall beside the condenser. Pull it, then kill the AC breaker in the panel too. Two isolation points, because the next step has you putting your hands where the charge is.
Safety — critical: Pull the disconnect at the condenser AND kill the breaker, then discharge the capacitor across a 20 kΩ / 2 W resistor before you touch a terminal. A run capacitor stores a lethal charge with the power off and holds it for hours — a screwdriver across the terminals will discharge it, damage it, and put an arc flash in your face. Use the resistor.
-
Discharge the capacitor, then photograph the terminals
Hold the resistor's leads across HERM–C, then FAN–C, for five seconds each. Then photograph the wiring before you pull a single spade. The can is stamped: HERM feeds the compressor and takes the larger value (45 µF here), FAN feeds the condenser fan and takes the smaller (5 µF), C is common and carries the line from the contactor.
-
Read the old can and match it exactly
µF must match — 45+5 replaces 45+5 and nothing else; a capacitor is tuned to its motor, and 5% off is a motor running hot for the rest of its life. Voltage is a CEILING, not a target: a 440V can replaces a 370V can safely, never the reverse. If you have a meter with a µF range, read the old one — more than 6% below its rating is failed.
-
Fit the new can and check your work
The strap on the unit is reused — the new can does not ship with one. Spades on per your photo, strap snug but not crushing the can, disconnect back in, breaker on. The fan should start on its own within a few seconds of the thermostat calling. Stand clear of the top of the unit while it starts.