Window Air Conditioner: Why are my window air conditioner's evaporator fins iced up?

The window air conditioner ambient thermistor-also known as a temperature sensor-senses the temperature of the air entering the air conditioner and communicates that temperature to the electronic control board, which cycles the compressor on and off to maintain the set room temperature. The ambient thermistor is attached to the evaporator coil that's behind the air filter, and a wire harness connects the ambient thermistor to the electronic control board. If the ambient thermistor is defective, the electronic control board will not cycle the compressor on and off properly to maintain the room temperature that is set on the control. Replace the ambient thermistor if it doesn't accurately measure the ambient room temperature.

The electronic control board regulates the compressor, evaporator fan and condenser fan in the window air conditioner. If the electronic control board doesn't activate those components properly, replace the control board.

Some window air conditioners have an electro-mechanical thermostat instead of an electronic control board to control the on/off cycling of the compressor and fans. Replace the thermostat if it fails to control the compressor and fans properly.

The evaporator thermistor is a temperature sensor that is mounted on the evaporator. It monitors the temperature of the evaporator to detect excessive frost on the evaporator. The thermistor communicates that temperature to the electronic control board through a wire harness connection. If the evaporator frosts up, the electronic control board will detect the frost according to the temperature communicated through the thermistor. If the thermistor is defective, the electronic control will not properly detect and eliminate the frost. Replace the evaporator thermistor if it doesn't properly detect the evaporator temperature.