The inside temperature sensor(in the controller) is installed on the air inlet of heater, it measures the air temperature at the air inlet, according to the temperature change the working state of the furnace and adjust the output of the thermal power. The outside temperature sensor is optional part which require extra configuration and can be put those heating places you want. Same working principle as inside temperature sensor . Name:Temperature Sensor For Air Parking Heater Type: Combustion Type Heater Model: NTC50K Plug: Square plug Line: 2 lines Fitment: For Ebespacher D2 D4 2000W 5000W Air 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 cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.