HVAC run/start capacitor supports motor starting and running. Related symptoms include humming, slow fan, protection trip or hot motor. Those symptoms do not prove the component failed: do not diagnose by appearance; qualified measurement is required. Check the thermostat's call, the breaker and the outdoor disconnect once.
What this covers
- AC, heat pumps and blowers
What it does not cover
Differential
Likely causes of “HVAC run/start capacitor”, and how to tell them apart
- 1
The thermostat, breaker or disconnect is stopping the unit
What separates it The unit is silent or the fan does not turn and the thermostat or the breaker was the cause, not a component.
How to confirm Check the thermostat's call, the breaker and the outdoor disconnect once.
- 2
A dirty coil or a blocked airflow makes the motor work harder
What separates it The unit is hot, the fan is slow and the coil or the filter is choked with dirt.
How to confirm Check the indoor filter and look at the outdoor coil for debris and blocked airflow.
- 3
The fan blade or motor bearing is stiff or seized
What separates it The fan hums but does not turn until pushed, and the sound is a steady hum from the outdoor unit.
How to confirm Do not push a fan blade by hand; this needs power off and a technician.
- 4
The capacitor has weakened or failed
What separates it The motor hums, starts slowly or trips the overload, and the capacitor may look swollen or leaking.
How to confirm Appearance alone does not prove it. Only a qualified measurement with the power off and the capacitor discharged can confirm it.
Before you touch a setting
Record this first — changing settings destroys the evidence
- The sound, whether the fan turns and whether the compressor starts.
- The age of the unit and any history of hard starting.
- The model and serial from the rating plate.
Try in this order
0 of 2 completedTest before you replace
ReversibleDo not diagnose by appearance; qualified measurement is required.
Weigh the repair
ReversibleReplacement difficulty for the hvac run/start capacitor is professional because charge may remain.
Stop boundary
When this stops being a self-service repair
- !A capacitor can hold a dangerous charge even with the power off. Do not touch or test it yourself.
- !Turn the unit off at the breaker and call a technician for humming, a hot motor or repeated overload trips.
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Can I diagnose a capacitor by looking at it?+
A bulging or leaking capacitor is likely bad, but a good-looking one can also be weak. Only a measurement by a qualified person is reliable, and it must be discharged first.
What does the capacitor do?+
It gives the motor a boost to start and helps it run. A weak capacitor is a common reason for a humming unit that will not start.
What are the signs?+
Humming, a fan that does not spin, or a unit that starts slowly.