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When your central air conditioner stops cooling, the cause is often airflow, refrigerant, or an electrical component. This guide covers the main checks you can do and when to call a professional. If your outside unit is not running at all, or your thermostat is not working, start with those symptom hubs.
| Symptom | Likely Cause |
|---|---|
| AC runs but air is warm | Filter, refrigerant, or capacitor |
| Outside unit hums but does not start | Capacitor or contactor |
| Breaker trips when AC runs | Breaker/overload or short circuit |
| Outside unit does nothing | Contactor, voltage, or wiring |
| Thermostat does not respond | Batteries or thermostat wiring |
Many "AC not cooling" problems stem from electrical components. Before assuming refrigerant or compressor failure, rule out these common electrical causes:
If your outside unit isn't running at all, the capacitor and contactor are the first electrical components to inspect.
A dirty filter restricts airflow, reduces cooling, and can cause the evaporator to freeze. Clean or replace the filter monthly during heavy use. This alone fixes many "AC not cooling" issues.
Ice on the evaporator coils blocks airflow. Causes: dirty filter, low refrigerant, fan not running. Turn off the AC, let ice melt (30–60 min), clean the filter, then try again.
Low refrigerant reduces cooling. Refrigerant handling requires certification—have a qualified tech check and recharge. Signs: AC runs but air is only slightly cool, coils frost over quickly.
Leaky ducts let cold air escape. Inspect duct connections. Blocked or disconnected ducts reduce delivered air. Ensure vents are open.
Call a licensed HVAC technician if the filter is clean and the problem persists, you smell burning, the breaker trips repeatedly, or you're uncomfortable with electrical or refrigerant work.
Clean the filter first. Top causes: dirty filter, low refrigerant, frozen evaporator, capacitor failure, contactor issues, or voltage problems.
Capacitor failure, contactor failure, breaker trips, low voltage, or thermostat wiring. See our electrical guides for each.
If the filter is clean and the problem persists, or you smell burning or see electrical issues, yes—call a licensed technician.
Most likely a failed capacitor. The compressor needs a startup boost—without it, the motor hums but does not spin.
Yes. A dirty filter restricts airflow, reduces cooling, and can cause the evaporator to freeze. Replace monthly during heavy use.
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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