[kaisen]

Coolant sensor could be EITHER a low-coolant-level OR over-temperature. I think the low coolant sensor would be a better bet. As the coolant levels rise and fall due to temps, it can fall just below the level where the sensor 'sees' coolant in the reservoir. If it is the 4.0L I6 please check for weeping coolant from the bottom of the water pump; it is very common and can cause a slow loss of coolant. Eventually the pump needs to be replaced. Here in the US you can get an indy tech to do the entire job for about $200.