Aircon Outdoor Run Capacitor
The run capacitor is a small electrical part in the outdoor unit. It is also one of the most common causes of compressor startup failure — and the most frequently misdiagnosed as the compressor itself.
What This Part Does
The compressor needs extra electrical support to start and maintain speed. The run capacitor provides that.
It stores energy and releases it in short bursts during startup and running. Without it, the compressor cannot start properly — even if the compressor motor is healthy.
Think of it as the start-assist for your compressor. If this part fails, the compressor fails to start. They share the same symptom.
How You Would Notice
The outdoor unit makes a humming sound, then clicks and stops. The fan may not spin at all, or may spin weakly.
The indoor unit keeps blowing air, but the room never cools down. The cycle may repeat — hum, click, stop, pause, hum again.
This exact pattern also describes a failing compressor. You cannot tell them apart by sound or symptom.
- Outdoor unit hums briefly, then cuts off
- Fan does not spin or spins slowly on outdoor unit
- Indoor airflow normal but room stays warm
It Might Not Be The Compressor
A weak capacitor and a failing compressor produce identical symptoms. The capacitor is a much cheaper fix.
Replacing the compressor first — without testing the capacitor — is one of the most common unnecessary repairs we see.
We test the capacitor before we go anywhere near the compressor.
How We Check
We measure capacitor value directly. A healthy capacitor reads within the rated tolerance printed on the label.
If the value is low, we replace the capacitor first and retest cooling before touching anything else.
If the capacitor tests fine, we move to compressor and board checks. We do not skip the order.
What We Find And What Happens Next
We replace the compressor only after confirming the capacitor is not the cause.
| Finding | Next Step |
|---|---|
| Capacitor value out of range | Replace capacitor, retest cooling |
| Capacitor fine, compressor not starting | Move to compressor diagnosis |
About The Repair
Capacitor replacement is a minor repair. The part is inexpensive and the job is straightforward.
If a technician quotes compressor replacement without measuring the capacitor first, ask what the capacitance reading was.
A confirmed fault has a number. If there is no number, there is no confirmed fault.
After Replacement
A replaced capacitor should restore normal startup immediately. If the outdoor unit still hums and stops after replacement, the compressor is the next thing to check.
Repeated hard starts from a weak capacitor can shorten compressor life over time. Catching a failing capacitor early is worth it.
When We Tell You To Wait
If the unit is still cooling — even if inconsistently — we will tell you that on-site.
A capacitor heading toward failure may give intermittent symptoms before full failure. That gives time to plan.
Our job is to give you an accurate picture. Not to find something to fix.