1999 2.4 A6 Avant. Water temp guage does not move (oil tempgauge is fine) ... is it likely to be the thermostat, guage or some sensor ? I'm thinking sensor .... if so where is it ?

Thanks in advance.

 

According to ETKA there are two possible temp sending units for your (year/model):
078 919 501 B (blue 4 pin unit)
078 919 501 C (black 4 ping unit)
Both are dual temp senders.

From the illustration it looks like its in the cooling pipe that runs across the rear of the engine to each bank, there seems to be two of them on top of the pipe.