Temperature sensor does not move

Hi there, I have just replaced the Thermostat in my P38 as the temperature guage was not reading anything. Turned out there was no a thermostat in there in the first place.

however having replaced it, there is still not reading. It moves about 1mm and that is it.

I have had my heat gun on it and the top hoses don't get hotter that 90 degrees.

Any ideas???

Have you thought of changing the sender unit?