Genuine Daikin 4P311238-1 Indoor Air Thermistor is the sensor that reads the temperature of the air entering the indoor unit, so it can judge the room and run to suit. When it starts to fail, the unit may stop holding the set temperature, run warmer or cooler than the room needs, or cycle on and off at odd times. A genuine sensor reads the air the way your Daikin expects, keeping it accurate — where a non-genuine part can read off and leave it running incorrectly.