have had a few of these lately mostly turned out to be not enough antifreeze(must be 50%) or the wrong type of antifreeze,the sensor doesnt actualy go in the coolant just in a tube in the expansion bottle and the density of the water makes a lot of difference to the sensor,check this first if you are 100% that both are correct then the sensor is up the shoot if u fit a new one cut the plastic cover off it carefully as it is stuck on quite well and sensor is easily broken