Just bought my A6 and discovered that the oil and engine temp gauges are under reading. (water temp might reach 70 rather than the 90 i'm expecting it to read when the engine is up to temp.) Its a Dec 2000 A6 avant 1.9 with 153,000 on the clock.

As both gauges are affected I'm guessing that it might be something in the instrument cluster, rather than the individual senders. Any help or thoughts would be greatly appreciated. Its a great car and I'm loving it :)

Thanks in anticipation. 

Your pressure and level sensors are likely to be 4 - 20mA (Current flow) type. With both being affected it could be a common fault or coincidental that both detecting devices are faulty at the same time. If it is a common fault I would first look at the instrument cluster wiring checking contacts, cleaning them and refitting them. The fuel and temp gauges are probably on a common ground (Guessing as do not have schematic) and if the resistance is high on the ground then this is your issue.

Good luck with your quest. 

Thanks for coming back. I was also erring on the problem being in the instrument cluster as its very unlikely both sensors would fail in the same way at the same time! I will have a go at checking and cleaning contacts as you suggested, once I feel brave enough and my Haynes manual has arrived !!

Personally ( from experience) i’d change your thermostat first, if its stuck open engine won’t get warm and so both oil and coolant temps will be down. Had my 1.9tdi a6 for 12 years now and its happened twice.