Fabia 1.9 TDI PD thermostat or coolant temperature sensor?

Hiy,

My Y reg fabia's coolant temperature hasn't reached 90 degrees for some time. It just stays at 45 goes up to 80 then drops down to minimum, it's not stable 90 as it used to be. What's the problem?

I've read quite a lot about coolant temperature sensor and thermostat and I don't know which one is to blame. Suppose the CTS is my first call, however I've never replaced it before and there is a worry I might end up with air in the system.

Is this job easy to do? and in the worst case is it difficult to drain coolant fluid and replace thermostat?

Regards,

Simon