What Is an Aircon Run Capacitor, and Why Does Cooling Fade?
A small electrical part in the outdoor unit that keeps the compressor or fan motor running at rated efficiency. When it weakens, cooling fades gradually; when it fails, the outdoor unit refuses to start — and both look like more expensive faults.
What the run capacitor does
The run capacitor is an electrical component that provides a continuous phase-shifted current to the compressor motor, the outdoor fan motor, or both, throughout the entire cooling cycle. Unlike the start capacitor which only operates during the first moment of startup, the run capacitor stays in the circuit the whole time the unit is running. It is what keeps the motor turning efficiently at rated speed. A motor operating without proper run capacitor support draws more current than it should, runs hotter, produces less output, and wears its windings faster. Because the motor still turns, the unit still cools — just worse than it should. This gradual degradation is the defining characteristic of a failing run capacitor, and it is what makes the fault easy to misattribute to refrigerant loss.
| Category | Electrical |
|---|---|
| Typical replacement cost | Varies |
| Replacement timeline | Same-day |
- Category
- Electrical
- Typical replacement cost
- Varies
- Replacement timeline
- Same-day
Run capacitor failure signs
What you observe, what causes it, and how a technician confirms or rules out each path.
| What you observe | Likely causes | How we verify |
|---|---|---|
| Cooling has faded gradually over weeks | Run capacitor losing rated value over time, Refrigerant slowly leaking | Measure capacitance with a meter and compare to the rated value on the component; a reading below tolerance confirms a weak capacitor. |
| Unit starts and runs but cannot reach set temperature | Compressor or fan motor running with weak capacitor support, Worn motor windings | Test capacitance and motor current draw together; a weak capacitor produces slightly higher than rated current for the output delivered. |
| Outdoor unit hums but does not start, then clicks and stops | Run capacitor failed completely, Seized compressor or motor bearing | Measure capacitance — a value near zero confirms the capacitor; if capacitance is in spec, check the motor shaft for free rotation. |
| High-pressure trips or shutdowns during hot weather runs | Outdoor fan underperforming from weak shared capacitor, Refrigerant pressures elevated by reduced condenser airflow | Test the fan capacitor and refrigerant pressures together; a slow fan with high head pressure points to the capacitor. |
- What you observe
- Cooling has faded gradually over weeks
- Likely causes
- Run capacitor losing rated value over time, Refrigerant slowly leaking
- How we verify
- Measure capacitance with a meter and compare to the rated value on the component; a reading below tolerance confirms a weak capacitor.
- What you observe
- Unit starts and runs but cannot reach set temperature
- Likely causes
- Compressor or fan motor running with weak capacitor support, Worn motor windings
- How we verify
- Test capacitance and motor current draw together; a weak capacitor produces slightly higher than rated current for the output delivered.
- What you observe
- Outdoor unit hums but does not start, then clicks and stops
- Likely causes
- Run capacitor failed completely, Seized compressor or motor bearing
- How we verify
- Measure capacitance — a value near zero confirms the capacitor; if capacitance is in spec, check the motor shaft for free rotation.
- What you observe
- High-pressure trips or shutdowns during hot weather runs
- Likely causes
- Outdoor fan underperforming from weak shared capacitor, Refrigerant pressures elevated by reduced condenser airflow
- How we verify
- Test the fan capacitor and refrigerant pressures together; a slow fan with high head pressure points to the capacitor.
How we verify a run capacitor fault
Diagnostic steps in order. Cheaper, more common causes get ruled out first so you do not pay for the wrong fix.
Measure the capacitor value with a capacitance meter and compare the result to the rated specification on the component.
Tools: Capacitance meter
Healthy reading: Reading sits within the manufacturer's tolerance band around the rated value.
Measure motor current draw under load to check for the elevated current pattern of a motor with weak capacitor support.
Tools: Clamp meter
Healthy reading: Current draw matches rated nameplate value for the operating conditions.
Confirm or rule out the capacitor before authorising refrigerant work — both faults produce gradual cooling decline on a unit that otherwise starts and runs.
Tools: Capacitance meter, Pressure gauge set
Healthy reading: Capacitance within spec; if so, pressure-test the refrigerant circuit as the next step.
Replacing the run capacitor
When replacement is the right call, when monitoring is fine, and when delay creates real risk.
Replace
If the capacitance reading is below the rated specification and the gradual cooling decline or motor underperformance matches.
You can wait
If the symptoms are intermittent on hot afternoons but the capacitor reading is still in spec — monitor and retest at the next service.
Do not wait
If the outdoor unit fails to start or keeps cutting out mid-cycle. Forcing a motor to start with inadequate capacitor support accelerates winding wear and shortens compressor lifespan.
If you proceed
Run capacitor replacement is one of the most straightforward and affordable outdoor unit repairs. The component is inexpensive and widely stocked across brands. Most technicians complete it within a single visit.
Some units have two capacitors supporting different motors. If one has failed, ask whether the other should be replaced at the same time. Before approving refrigerant work on a unit with gradual cooling loss, confirm whether the run capacitor was tested and what the meter reading showed.
Frequently asked questions about the aircon run capacitor
What are the warning signs of a failing run capacitor?
When should I switch off the aircon because of the run capacitor?
How does a technician confirm the run capacitor has failed?
Does the run capacitor need replacing, or can it be repaired?
What is the run capacitor responsible for?
Describe your situation
Describe the cooling pattern and outdoor unit behaviour.