Ive a c max 1.6 tdci 04 plate. Recently when driving over 50mph the temp gauge shoots up to max temp 120degrees. The car then looses power as it goes into limpmode. On reducing my speed to under 50mph the gauge goes back down again to half way. I lifted the bonnet up (im a girl i thought it best i did so!) It did not seem that hot to me. This happened a few times so took it to a mechanic friend. He changed the sensor in thermostat and then the whole thermostat. Still no joy, its still doing it. The diagnostic machine came up with nothing also.

Ive also noticed that as soon as i start driving even at low speed the temp gauge goes up to half way almost immedietly and fan kicks in. previously before all this it would go to 1/4 way up.

Wondered if anyone had any ideas.
Thanks

Can you get the heater to blow warm air and are you certain the cooling system is full of water and that you don't have an air lock somewhere

[It turned out to a crack in the radiator. Tho the water level remained constant. Odd. Had it done on the warrenty so that was good, Thanks for your ideas though, Anna.