Genuine Daikin 4019008 Indoor Coil Thermistor is the temperature sensor clipped to the indoor coil that tells the unit how warm or cold the coil is, so it can run at the right level. When it starts to fail, the unit may stop holding the set temperature, blow warmer or cooler than it should, or switch on and off at odd times. A genuine sensor reads temperature the way your Daikin expects, keeping the unit's response accurate — where a non-genuine part can read off and leave it running incorrectly.