Q:
A: The low cost trial solution is to replace the thermostat. It may be too old and too slow in opening and closing, letting the water get too hot, then opening late. Too hot makes the STOP light come on. Also, how long has it been since you replaced the coolant? That old mixture could be approaching the boiling point too easily. Try changing that too. It's not an expensive trial solution either. 3 DAYS LATER: I just thought of something else: The water temperature sensor for the 205 Gti, the 306, and the 405 are all found in the water thermostat housing. Look at your 406, where the upper radiator hose connects to the engine block. You should/might see an electric sensor with two wires screwed into the aluminium housing. This sensor is removeable with a spanner (you will loose 1 litre coolant) and can also be replaced. cost?? This might be a faulty sensor, giving readings intermittantly to the dashboard guage... and your temperature is really OK, always 90 C or thereabouts? You can do this job yourself in the driveway. If I'm wrong, you have spent 5 to 10 pounds, and one hour. If you coolant is relatively new, then save your coolant and reuse it.