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This guide explains what a contactor does, symptoms of failure, how to inspect safely, repair costs, when to replace vs repair, and how it ties into capacitor and breaker issues.
Think of the contactor as a heavy-duty relay switch. When your thermostat calls for cooling:
If the contactor fails, power never reaches the compressor, the outside unit won't start, and the system blows warm air. See our AC not cooling pillar guide for the full diagnostic flow.
See also: AC Not Cooling Guide, HVAC Capacitor Guide, AC breaker guide.
Most common causes:
Contactors are mechanical parts—they wear out. Typical lifespan: 5–10 years. Thermostat wiring faults can also prevent the contactor from receiving the 24V signal.
⚠️ High voltage present. Proceed carefully.
Do NOT manually press contactor while energized.
Technically yes. Part cost: $20–$80. Professional replacement: $150–$350.
Risks: incorrect wiring, misdiagnosis (compressor may be real issue), electrical shock. If unsure → hire licensed HVAC tech.
| Symptom | Likely Cause |
|---|---|
| Humming but no spin | Capacitor |
| Clicking but no power | Contactor |
| Immediate breaker trip | Compressor or short |
| Unit won't shut off | Stuck contactor |
These components often fail together. See HVAC Capacitor Guide.
Replacing a worn contactor early prevents expensive damage.
Call a licensed technician if:
Electrical failures escalate quickly.
| Repair Type | Typical Cost |
|---|---|
| DIY Part Only | $20–$80 |
| Professional Replacement | $150–$350 |
Yes. Ants frequently infest contactors and cause shorts.
5–10 years on average.
No—relatively inexpensive compared to compressor damage.
Likely a failed contactor or capacitor.
Humming but no spin = capacitor. Clicking but no power = contactor. Both can fail together.
Final authority takeaway: If your outdoor unit clicks but does not start, or stays running nonstop, the contactor is one of the first components to inspect. It is inexpensive, common, and critical to system function. Diagnose intelligently—or call a professional before damage spreads.
If your HVAC system is experiencing electrical issues and you're unsure how to proceed, schedule a licensed inspection before further damage occurs.
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Last updated: March 2026 · Reviewed for technical accuracy
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