Outside Unit Not Running: Electrical Diagnosis

Capacitor, contactor, breaker, voltage. When the outdoor AC unit won't start.

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  If your outdoor AC unit won't start, the cause is usually electrical. The condenser fan and compressor need power from the contactor, a startup boost from the capacitor, and a clear signal from the thermostat. When any of these fail, the outside unit stays silent.

This guide covers the most common causes when your outside unit does not turn on, and links to detailed electrical diagnosis guides.

Quick Diagnosis Table

Symptom	Likely Cause
Unit hums but fan does not spin	Failed capacitor
Clicking but no startup	Failed contactor
Complete silence	Tripped breaker, low voltage, or wiring
Thermostat calls for cool—nothing happens	Thermostat wiring or contactor

Pre-Diagnosis Checklist

✔ Thermostat set to cool, temp below room temperature
✔ Check electrical panel—AC breaker on
✔ Outdoor disconnect switch closed
✔ Listen for humming (capacitor) vs clicking (contactor) vs silence (power/wiring)
✔ If unsure, start with AC not cooling pillar for full flow

Top Causes: Outside Unit Not Running

Failed capacitor — If your system hums but does not start, the most common cause is a failed capacitor (see our complete HVAC capacitor troubleshooting guide).
Failed contactor — If you hear clicking but the fan does not spin, the contactor may be stuck or burned. See HVAC contactor guide.
Breaker tripped — Check the electrical panel. If the AC breaker keeps tripping, see our AC breaker diagnostic guide.
Low voltage — Voltage below safe range prevents the compressor from starting. See HVAC voltage requirements.
Thermostat wiring — Loose or broken low-voltage wires can prevent the cooling signal from reaching the outdoor unit. See HVAC wiring basics.

If your AC is not cooling at all, start with our AC not cooling pillar guide for the full diagnostic flow.

Frequently Asked Questions

Why won't my outside AC unit turn on?

Most often a failed capacitor, contactor, or tripped breaker. See our electrical guides for diagnosis.

Is it the capacitor or contactor?

Humming but no spin = capacitor. Clicking but no start = contactor. Both can fail together.

Can low voltage prevent the outside unit from starting?

Yes. Voltage below 198V (240V systems) or 108V (120V systems) can prevent the compressor from starting.

What if the thermostat works but the outside unit does nothing?

Check wiring between thermostat and outdoor unit, then capacitor and contactor. Wiring faults are common.

Related Electrical Guides

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About the Author

Adam Hall — Founder, DecisionGrid

DecisionGrid's technical guides are written and reviewed using:

System-level electrical analysis
Real-world RV troubleshooting patterns
Manufacturer documentation review
Field-tested diagnostic workflows

Our goal: Clear, structured troubleshooting — not guesswork.

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Updated March 2026 · Reviewed for technical accuracy

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Last updated: March 2026 · Reviewed for technical accuracy

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