5 Signs Your AC Capacitor Is Failing (And What to Do About It)

What a Capacitor Actually Does

Your AC system has two main capacitors — a start capacitor and a run capacitor (or a dual-run capacitor that handles both). They store and release electrical energy to:

1.     Give the compressor motor a torque boost to get started (start capacitor)

2.     Keep the compressor and condenser fan running smoothly under load (run capacitor)

Capacitors are measured in microfarads (µF) and voltage rating. A failing capacitor causes symptoms that range from reduced efficiency all the way to a system that won't start at all.

Sign 1: The AC Won't Start — You Hear a Hum But No Startup

This is the classic dead capacitor symptom. You hear the system try to start — a hum or click — but the compressor or fan doesn't spin up. After a few seconds, the breaker may trip or a thermal overload may shut it down.

The start capacitor provides the initial surge needed to get the motor turning. Without that surge, the motor draws locked rotor amperage trying to start, heats up fast, and the overload trips.

Sign 2: The Condenser Fan Spins Slowly or in the Wrong Direction

A weak run capacitor can't maintain the phase shift the fan motor needs to run at full speed. The fan may spin at half speed, overheat the condenser, and cause the whole system to run inefficiently.

In some cases with a shorted capacitor, the fan can actually reverse direction. If your condenser fan is spinning but not moving much air, test the capacitor.

Sign 3: The System Runs But Struggles on Hot Days

At high ambient temperatures, the compressor needs more starting torque and the capacitor is working harder. A marginal capacitor that holds up in mild weather may fail when it's 95°F outside.

This seasonal pattern — system works fine in spring but struggles in summer heat — is a strong indicator of a capacitor that's degraded but not fully failed.

Sign 4: The Capacitor Is Visibly Bulging or Leaking

Capacitors have a pressure relief vent on top (it looks like a small X or star). When the dielectric fluid inside breaks down, pressure builds up and the top bulges or leaks oil.

If the top of your capacitor is dome-shaped instead of flat, or if you see brown residue around it, it's failed and needs immediate replacement. This is one of the easiest field diagnoses in HVAC.

Sign 5: High Electric Bills With Normal Runtime

A degraded run capacitor forces the compressor to draw more current than it should. The system still runs and cools, but it's working inefficiently. If your energy bills have crept up without a change in usage patterns, the run capacitor is worth testing.

How to Test and Replace a Capacitor

Safety critical: capacitors store a lethal charge even when power is off. Always discharge the capacitor through a 20,000-ohm, 5-watt resistor before touching terminals.

To test: set your multimeter to capacitance mode (µF). A good capacitor should read within 6% of its rated value. More than 10% low means replace it.

To replace: match the µF rating and voltage rating exactly. Dual-run capacitors have three terminals — HERM (compressor), FAN (condenser fan), and C (common). Match the ratings for each section.

At HVAC Surplus, we stock dual-run capacitors in the most common residential ratings — 35/5, 45/5, 45/7.5, and more, at outlet prices.