I went on a 160 mile cruise through some twisties and the desert yesterday, and my low coolant light came on and now it comes one about one minuite after everytime I start the car. The first time I did it, I couldnt see any water in the resivior, but I put some in and now it stays almost full but the light is still on. I think I heard someone once who had a sensor go out in it. could that be it or is it something more serious?