What Is an Aircon Start Capacitor, and Why Won't It Spin?
A short-acting electrical component that gives the compressor or fan motor the jolt of starting torque needed to spin from a standstill. When it fails, the motor hums but cannot get off the ground — and looks identical to a seized compressor.
What the start capacitor does
The start capacitor is a short-acting electrical component in the outdoor unit. It gives the compressor or fan motor the extra push needed to start from a standstill. Once the motor reaches speed, the capacitor drops out of the circuit. On some units, a start relay disconnects the capacitor after startup; on others, the capacitor handles the transition on its own. Either way, it only works in the first fraction of a second of each startup attempt.
| Category | Electrical |
|---|---|
| Typical replacement cost | Varies |
| Replacement timeline | Same-day |
- Category
- Electrical
- Typical replacement cost
- Varies
- Replacement timeline
- Same-day
Start 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 |
|---|---|---|
| Outdoor unit hums but does not spin up | Start capacitor lost its rated value, Open or shorted capacitor | Measure capacitance with a meter — a value well below the rated specification confirms a failed start capacitor. |
| Startup attempts fail after a brief hum or click | Insufficient starting torque from weak capacitor, Start relay fault | Test the capacitor first; if reading is in spec, check the start relay and motor windings. |
| Breaker or overload trips after each restart attempt | Locked-rotor current from a stalled motor, Wiring fault under starting load | Do not force repeated restarts — measure capacitance once power is safely isolated to confirm or rule out the capacitor. |
| Problem is more frequent in hot weather or after long idle periods | Higher starting torque required at elevated pressures, Marginal capacitor that is failing under load | Test capacitance at room temperature and compare against rated value; weakening shows even when the unit appears to run on cooler days. |
- What you observe
- Outdoor unit hums but does not spin up
- Likely causes
- Start capacitor lost its rated value, Open or shorted capacitor
- How we verify
- Measure capacitance with a meter — a value well below the rated specification confirms a failed start capacitor.
- What you observe
- Startup attempts fail after a brief hum or click
- Likely causes
- Insufficient starting torque from weak capacitor, Start relay fault
- How we verify
- Test the capacitor first; if reading is in spec, check the start relay and motor windings.
- What you observe
- Breaker or overload trips after each restart attempt
- Likely causes
- Locked-rotor current from a stalled motor, Wiring fault under starting load
- How we verify
- Do not force repeated restarts — measure capacitance once power is safely isolated to confirm or rule out the capacitor.
- What you observe
- Problem is more frequent in hot weather or after long idle periods
- Likely causes
- Higher starting torque required at elevated pressures, Marginal capacitor that is failing under load
- How we verify
- Test capacitance at room temperature and compare against rated value; weakening shows even when the unit appears to run on cooler days.
How we verify a start 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 it against the rated specification printed on the component body.
Tools: Capacitance meter
Healthy reading: Reading sits within the manufacturer's tolerance band around the rated value.
Listen for the startup hum pattern and check whether the motor shaft turns freely by hand.
Healthy reading: Shaft rotates freely with no grinding or resistance, indicating no bearing seizure.
Test the capacitor before concluding the compressor or motor has failed — a stalled motor draws high locked-rotor current that mimics an electrical fault.
Tools: Capacitance meter, Clamp meter
Healthy reading: Capacitor in spec; motor draws normal starting current and ramps up cleanly.
Replacing the start capacitor
When replacement is the right call, when monitoring is fine, and when delay creates real risk.
Replace
If the capacitance reading is below rated specification and the startup failure pattern matches — motor hums, does not spin, and the problem repeats across restarts.
You can wait
If startup was marginal once but the unit now runs normally — schedule a capacitance check at the next service visit.
Do not wait
If the unit hums and fails to start. Each stalled startup passes high current through the motor windings and accelerates wear. Turn the unit off and arrange diagnosis.
If you proceed
Start capacitor replacement is one of the least expensive outdoor unit repairs and is typically completed within a single visit. The component is standard across most brands, so availability is rarely an issue.
Before approving a compressor replacement on a unit that hums but will not start, confirm whether the start capacitor was tested and what the meter reading showed. A capacitor costs far less than a compressor — skipping the test is a common and avoidable error.
Frequently asked questions about the aircon start capacitor
What are the warning signs of a failing start capacitor?
When should I switch off the aircon because of the start capacitor?
How does a technician confirm the start capacitor has failed?
Does the start capacitor need replacing, or can it be repaired?
What is the start capacitor responsible for?
Describe your situation
Describe the startup sound and pattern.