my temperature gauge has suddenly started reading high although the engine is not really running hot. the indicator never drops to zero, and when the engine warms up, it shows about one quarter over normal. is the sensor bad?

 

clean the contacts to the sensor first, when did you do a coolant service??

 

This is most likely the classic symptoms from the defective instrument cluster...

http://www.classcounsel.com/news/audi.html

Better get on the horn to Audi and start asking them to pay up. They agreed to take care of all the UK owners that had any issue. Its bad news. Eventually the whole cluster dies and then the car won't start because the security system is dependent on components in the instrument cluster.