I've just recently bought an a4 1.8t quattro saloon 2003
The engine management has come on and the fault code is p0117(coolant temp sensor)
On a morning when I start the car after 10secs the fan comes on and eventually goes off after 5mins.
Driving the car feels fine and the temp gauge goes upto 90degrees then doesn't move after,although the when the car is hot and been stood in traffic the fan doesn't kick in although it's not overheated yet(maybe I've not been stuck or sat long enough).
But I did start it up this morning and the coolant warning light came on so I checked the coolant level and it had all gone,this was the first time all the coolant had gone.....is it true this could just be the coolant temp sensor that require changing?

 

You should replace the coolant temp sensor and make sure you don't have any coolant leaks coming from your coolant flange.