Symptoms
- Outdoor unit hums but the fan or compressor doesn't start
- Fan spins slowly or needs a push to get going
- AC works intermittently, sometimes starting after several tries
- Capacitor case looks swollen or bulging if visible
Likely causes
- CommonCapacitor worn out from age and heat exposureCapacitors naturally degrade over years of heat cycling in an outdoor unit.
- PossiblePower surge damageA lightning strike or grid surge can damage a capacitor suddenly.
- PossibleOverheating from a dirty or poorly ventilated outdoor unitExcess heat shortens capacitor lifespan and can cause premature failure.
What to do
- Turn off power to the outdoor unit at the disconnect before inspecting anything.
- Visually check the capacitor (a cylindrical component, if accessible) for bulging, leaking, or a domed top, which are clear signs of failure.
- Note the exact symptom: a hum with no movement, a slow-spinning fan, or intermittent starting all point toward the capacitor.
- Don't touch or attempt to test the capacitor yourself, since it can hold a dangerous electrical charge even with power off.
- Call an HVAC technician to test and, if needed, replace the capacitor with the correct rated part.
- Avoid running the system repeatedly while a capacitor is suspected to be failing, since this can strain and damage the compressor.
Tools and parts
AC dual run capacitorSearch: “air conditioner dual run capacitor correct rating”
When to call a pro
Call an HVAC technician to safely test and replace a failing capacitor with the correctly rated part.
Safety
Use caution
Capacitors can hold a dangerous electrical charge even after power is disconnected; never touch or test one yourself.