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.