You'll know when it comes on when you hear it. It usually kicks in about 10 to 30 seconds after you cold start the car. You'll hear a higher pitch whirring almost like a blower motor gone bad or a small jet firing up its engine. (kinda cool sound actually) The other way to know its on is to get yourself an OBDII scanner. The scanner should be able to pick up on the signal and tell you when the circuit kicks in