Every summer, millions of homeowners call an HVAC technician because their AC stopped working. The diagnosis? "Bad compressor — you need a new system." Cost: $2,500-$4,000.
But here's what many homeowners don't know: 60-70% of "dead compressor" calls are actually a failed run capacitor — a $15 part that takes 10 minutes to replace.
What Is a Run Capacitor?
The run capacitor is a small cylindrical component in your outdoor AC unit. It stores electrical energy and provides the extra boost your compressor and fan motor need to start and run efficiently. Think of it like a battery that helps your engine turn over.
When it fails, the compressor hums but won't start, or the fan motor spins slowly. To an untrained eye, it looks like the compressor itself is dead.
How to Check Yours in 60 Seconds
Look at your capacitor through the AC unit's access panel (power OFF first):
- Swollen or bulging top? → It's dead. Replace immediately.
- Oil leaking from the bottom? → It's dead.
- Rust or corrosion? → Failing soon.
- AC hums but won't start? → 90% chance it's the capacitor.
Use our free AC Compressor Troubleshooter for a step-by-step diagnosis.
The Math That Should Make You Angry
| Option | Cost | Time |
|---|---|---|
| Replace capacitor yourself | $15-$45 | 10 min |
| Tech replaces capacitor | $150-$300 | 30 min |
| Tech says "bad compressor" | $2,500-$4,000 | Days |
Always check the capacitor before agreeing to a compressor replacement. A reputable tech will test it first. If they don't, get a second opinion.