Genuine Daikin 1998939 Outdoor Air Thermistor is the sensor that reads the outdoor air temperature, giving the system the outside reading it uses to run correctly. When it starts to fail, the outdoor unit may lose output, run hot or cold for conditions, or stop and start at odd times. A genuine sensor reads the air the way your Daikin expects, keeping the system accurate — where a non-genuine part can read off and leave it running incorrectly.