10 Common Reasons Your AC Won’t Start—and How to Solve Each One

10 Reasons Your Air Conditioner Won’t Start — and How to Get It Running Again

Nothing is more frustrating than a silent air conditioner on a scorching day. If your AC refuses to turn on, the cause could be as simple as a loose switch or as complex as a failing motor. Below are ten of the most common culprits and what you can do about them.

1. Thermostat Problems

The thermostat tells your AC when to cool. Dead batteries, loose wiring, or miscalibrated sensors can all prevent it from signaling the system to start.

What to do: Replace the batteries, confirm that the thermostat is set to “cool,” and check that the temperature is lower than the room reading. If that doesn’t help, have the thermostat inspected or replaced.

2. Power Supply Issues

If the AC has no electricity, it can’t run. A tripped circuit breaker, blown fuse, or loose power connection may be stopping current from reaching the unit.

What to do: Check your electrical panel and reset any tripped breakers. If the breaker trips again or fuses repeatedly blow, call a licensed technician to locate the root cause.

3. Clogged or Dirty Air Filters

Dust-filled filters restrict airflow and can cause the system to overheat or shut down to protect itself.

What to do: Replace disposable filters or wash reusable ones every one to three months to keep air moving freely and reduce strain on the system.

4. Faulty Start or Run Capacitors

Capacitors store the electrical charge that starts the AC’s motors. If one burns out or weakens, the compressor and fans can’t begin operating.

What to do: A professional should test and replace any failed capacitors—these components hold high voltage and can be dangerous to handle.

5. Low Refrigerant or Leaks

Refrigerant moves heat out of your home. Too little refrigerant or a leak can stop cooling entirely and may damage the compressor.

What to do: Have a certified HVAC technician locate and repair leaks and recharge the system. Refrigerant must be handled with proper equipment and licensing.

6. Frozen Evaporator Coils

When airflow is blocked or refrigerant is low, the evaporator coil can freeze into a block of ice, halting cooling.What to do: Turn off the unit and allow it to thaw. Replace or clean filters and call a professional to diagnose airflow or refrigerant issues before restarting.

7. Malfunctioning Fan Motor

The fan motor circulates air across the evaporator coil and through your ducts. If it stops, the system cannot distribute cool air.

What to do: An HVAC technician can test the motor, lubricate moving parts, or replace the motor if it has failed.

8. Damaged or Loose Wiring

Corroded connectors, frayed wires, or loose terminals interrupt the power supply and can keep the AC from starting.What to do: Electrical repairs are not DIY-friendly. Have a licensed technician check and replace wiring to ensure safe operation.

9. Faulty Contactor or Relay Switch

The contactor (or relay switch) is the electrical gateway that powers the compressor and fans. If it fails, the system never receives the signal to start.

What to do: Only a professional should replace a bad contactor or relay, as it involves high-voltage components.

10. Aging or Burned-Out Compressor

The compressor is the heart of the cooling system. Over time, wear and tear can lead to failure, leaving the AC completely inoperative.

What to do: If the compressor is failing, a major repair or full system replacement may be necessary. A technician can advise on the most cost-effective solution.

Bottom Line

An AC that won’t power on doesn’t have to mean days of discomfort. Start by checking simple fixes like the thermostat, breaker, and filter. For more complex issues—such as refrigerant leaks, electrical faults, or failing motors—call a professional. Regular maintenance and early attention to unusual noises or reduced airflow are the best ways to keep your system reliable and your home cool all season long.