Demon is right. In adition to the fact that the 'tour likes to have the coolant above the 'max' line, there is an optical sensor that can go bad. THe way I understand it is that if you collant gets a bit gunky, or a film developers on the inside of the while plastic coolant resivior, the coolant light can also be tripped. A good cleaning of the resivoir might also solve some fo the problems.
