Start and run capacitors give single-phase motors the extra electrical push to start and the efficiency to keep running. A weak or failed capacitor can cause humming without starting, slow fans, hot motors and tripped overloads. They store a charge even when power is off, so only a technician should handle them.
Capacitors are cheap, common and often blamed correctly. A weak one can make a healthy compressor look dead, and a failed one on a fan motor can make the box warm even though the compressor runs.
What it does
A start capacitor gives a burst of energy for the first seconds so the motor turns against pressure. A run capacitor stays in the circuit to keep the motor efficient and cool. Fan motors often have a run capacitor only. Capacitors are rated in microfarads and volts, and the replacement must match closely.
Signs it is failing
- Compressor hums and does not start.
- Fan motor spins slowly or needs a push.
- Motor runs hot or trips on overload.
- Bulging, leaking or swollen capacitor case.
- Hard starting that gets worse in hot weather.
- Clicking sounds from the contactor with no start.
Common causes of failure
| Cause | What happens |
|---|---|
| Age and heat | The internal film degrades |
| Voltage spikes | Storms and surges shorten life |
| Overload from a dirty coil | High current stresses the part |
| Wrong rating | A replacement that does not match causes stress |
| Manufacturing defect | Early failure in a new part |
What you can safely check
- Look, do not touch. With power off at the disconnect, note any bulging from outside the enclosure if visible.
- Note the symptom. Humming, slow fan or trip on start.
- Check fans by sight. Is the motor turning? Any hesitation?
- Record the pattern. Time of day and outdoor temperature.
A capacitor can hold a dangerous charge for a long time after power is removed. Do not touch terminals or try to discharge one. Leave testing and replacement to a technician.
What a technician does
- Discharges and tests capacitance with a meter.
- Compares readings with the rating on the case.
- Replaces with a matching capacitor.
- Checks motor windings and current draw.
- Looks for the reason it failed, such as heat or surges.
Keeping it healthy
- Keep condenser coils clean so motors do not overwork.
- Consider surge protection where storms are common.
- Have capacitors tested at yearly service.
- Keep enclosures dry and cool.
Capacitors often fail in the first hot weeks of summer. A test in spring can catch weak ones before they leave you with a warm box.
When to call for service
Call quickly if the compressor hums without starting or a fan motor will not turn, especially with food inside. A struggling motor can burn out quickly, and repeated resets make it worse. Note the outdoor temperature and when the problem began.
Repair or replace
Capacitors are replaced, never repaired. They are inexpensive, but a wrong replacement can damage a motor, so the rating must match. If the same capacitor keeps failing, look for heat, surges or a struggling motor. See surge protection for prevention.
Related reading: the parts and components guide, condenser fan not spinning and evaporator fan not working.
Questions about this part
Can I test a capacitor myself?
Not safely without training. They store dangerous charge.
Why do fan motors need a capacitor?
Single-phase fan motors use a run capacitor for smooth, efficient operation.
What does a bulging capacitor mean?
It has failed or is about to. Replace it soon.