Genuine Daikin 4009510 Outdoor Thermistor is the temperature sensor in the outdoor unit that gives the system a reading it uses to run correctly through the cycle. When it starts to fail, the outdoor unit may lose output, run hot or cold, or stop and start at odd times. A genuine sensor reads temperature the way your Daikin expects, keeping the system accurate — where a non-genuine part can read off and leave it running incorrectly.