The Short Version

An air conditioner that never cycles off is working harder than it should. That means higher energy bills, more wear on the compressor, and — if left unchecked — a shorter system lifespan. The good news: most causes are fixable without replacing the unit.

5 Reasons Your AC Won't Shut Off

1. Dirty Air Filter

A clogged filter restricts airflow across the evaporator coil. The system can't absorb enough heat, so it keeps running trying to reach the setpoint. Check your filter first — it takes two minutes and costs nothing.

2. Dirty Evaporator or Condenser Coils

Coils covered in dust or debris can't transfer heat efficiently. The system compensates by running longer. Annual tune-ups prevent this.

3. Low Refrigerant

If your system is low on refrigerant (usually from a slow leak), it loses cooling capacity. The compressor runs continuously trying to make up the difference. You'll also notice ice forming on the refrigerant line near the air handler.

4. Undersized System

An AC that's too small for the space it's cooling will run all day on a hot Chicago afternoon and never fully satisfy the thermostat. This is a design problem — not a maintenance issue.

5. Thermostat Issues

A faulty thermostat may not signal the system to shut off even after the setpoint is reached. Try setting the thermostat 5°F above the current room temperature. If the system doesn't shut off within a few minutes, the thermostat may need replacement.

When to Call a Technician

If you've replaced the filter and the system is still running constantly, schedule a diagnostic visit. A technician can check refrigerant charge, measure airflow, inspect coils, and test the thermostat — all in one visit.