Window Air Conditioner: Running but not cooling If the window air conditioner's compressor and fans are running but the air conditioner isn't cooling, possible causes include a failed compressor or the lack of refrigerant due to a leak in the sealed system. If the compressor isn't running but the fans work, the start relay or compressor might have failed. A defective electronic control board or control thermostat could prevent the window air conditioner from cooling. See the repairs listed below for help with correcting this problem. Replace the window air conditioner compressor The compressor is a pump that compresses the refrigerant—Freon—that’s sealed in the window air conditioner. If the compressor doesn’t run when activated, have a refrigerator service technician replace it; by law, an EPA-licensed technician must do this repair because it involves Freon. Replace the window air conditioner evaporator thermistor The evaporator thermistor is a temperature sensor that is mounted on the evaporator.