Engine temperature

[Obelix]

Engine temperature problem.
The temperature gauge is displaying different temperature depending on the weather. On a cold day it can take 45mm (+) on a cold day, and will get to 80° in 10mn on a normal day. I've had a mechanic looking at it but it's never been cold enough for them to see any problems.
I suspected a dodgy thermostat, but that has been ruled out.
This is my 3rd Vito. (2012) and I've never come across this problem before.
Any suggestions as to what could cause this?
Thanks

 

[Bobby Dazzler]

My first thought was that the thermostat is failing but you say that that has been ruled out - how did you do that?

Thermostats are usually fairly inexpensive and relatively straightforward to replace, so unless there’s something more obviously at fault then I would replace the thermostat.

 

[BlackC55]

What engine do you have?