I was putzing around with the windshield washer level sensor on my '99 A6 today (the tank was leaking at that spot). When I was done, the fuel level and engine temperature gauges no longer worked. No clue how those things can possibly be related.

According to the wiring diagram, it looks like the windshield washer level sensor goes straight to something called the "Auto Check System". Is that part of the instrument cluster? Did I fry it by accident?

 

I checked with VCDS, and the instrument cluster does not read the correct values from the fuel level and coolant temp sensors. So I took the gauge cluster out and checked the resistances at the appropriate pins on the blue connector (5/7 for fuel level, 7/8 for coolant temp) and they're exactly where they should be. So the sensors work fine, but the instrument cluster no longer reads them. Guess the instrument cluster is damaged... time to get it rebuilt

 

The gauges will act erratically if the ignition switch is failing. Don't know about the '99 but up through '95 (the C4 Chassis and B4 Chassis) this was a common problem. The switch is $35 from the dealer (only use OEM on this). Install can vary from an hour to 10 minutes depending on how many times you have done it.