Mine always comes on regardless of engine temp. I've owned two vw rabbits, and there was some kind of temp sensor which cut off the glow plug circuit when the engine was warm=no glow plug light. Seems like this system would be beneficial for longer glow plug life since the plugs will not operate as much. I did buy a set of wratchet wrenchs at a truckload tool sale for under $20, so at least when it is time to replace the glow plugs it will take much less time.