AC Contactor vs Capacitor: What's the Difference and Which One Failed?

June 28, 2026 • CORALVU LIMITED COMPANY

Two Different Parts, Similar Symptoms

When your AC stops working, the two most common culprits are the contactor and the capacitor. They're both in the outdoor condensing unit, they're both relatively cheap to replace, but they do completely different jobs. Knowing which one failed saves you time and money.

What Does a Capacitor Do?

Think of the capacitor as a battery that gives the motor a boost. It stores electrical charge and releases it to start the compressor and keep the fan motor running smoothly. There are two types:

  • Start capacitor — provides the initial surge to get the motor spinning
  • Run capacitor (dual run) — maintains consistent power to both the compressor and fan motor during operation

What Does a Contactor Do?

The contactor is basically a heavy-duty relay switch. When your thermostat calls for cooling, it sends a 24V signal to the contactor, which closes the circuit and allows 240V power to flow to the compressor and fan. It's the gatekeeper — if it fails, nothing gets power.

How to Tell Which One Failed

It's probably the CAPACITOR if:

  • The outdoor unit hums but the fan doesn't spin
  • You can spin the fan manually and it starts running
  • The unit runs briefly then shuts off
  • The capacitor looks swollen or is leaking oil

It's probably the CONTACTOR if:

  • The outdoor unit is completely dead — no hum, no click, nothing
  • You can hear a faint click from the contactor but nothing turns on
  • The contactor has visible pitting, burn marks, or melted plastic
  • Ants or debris are stuck between the contact points
  • The unit runs continuously and won't shut off (contacts welded closed)

Cost Comparison

Both parts are affordable for DIY replacement:

  • Capacitor: $15–$35 for the part, 15 minutes to install
  • Contactor: $25–$45 for the part, 20 minutes to install
  • HVAC tech service call: $150–$400 for the same job

When to Replace Both

If your system is 5+ years old and one fails, it's smart to replace both at the same time. They wear at similar rates, and you're already in there with the power off. Total parts cost under $60 beats a second service call six months later.

CoralVolt carries both — UL certified contactors and capacitors, rated for heavy-duty commercial use but priced for everyone.