It is a temperature sensor. If you have it you can use the temperature slide bar to control the heat coming out of the heater. If you don't the only way you can control the heat is with the fan speed. I've had it both ways. It works both ways but better with the sensor. It is a pain to get installed properly so a lot of guys just cut it off and toss it. If I had it to do over I'd take the lots of extra time and do it right.