The inside temperature sensor(in the controller) is installed on the inlet of heater, it measures the temperature at the inlet, according to the temperature change the working state of the furnace and adjust the output of the thermal . The outside temperature sensor is optional part which require extra configuration and can be put those heating you want. working principle as inside temperature sensor . Name:Temperature Sensor For Parking Heater Type: Combustion Type Heater : NTC50K Plug: plug Line: 2 lines Fitment: For Ebespacher D2 D4 2000W 5000W Parking Heater Colour: Blue Line Material: copper wire + plastic Package Contents: 1 * Temperature Sensor Only the above package content, other products are not included. Note: Light and different displays may the color of the item in the a little different from the real thing. The measurement allowed error is +/- 1-3cm.