Have the same issue with my 4.2 but it somewhat works. No codes and the sensors all read fine. Now I'm locked out and can't access the instruments section of vcds. I think I have to disconnect the battery to fix it.
Yes I've had the same thing on my W12, although mine doesn't show empty all the time, it will only show empty if the tank is more than half full, half full and under it reads but is a bit out, I've run out a few times with it showing just above reserve. It's apparently the wiring loom, I've read it somewhere on here that it's basically a bad/broken wire and my garage also did some research and has done another with the same problem. Trouble is replacing the whole loom is a big job, they are seeing if it can be 'fixed' where you don't have to replace the whole loom but just the one wire instead.
I just checked all sensors again. The lower sensors have a resistance from 50 to 1000 Ohm. I mentioned 50 to 300 Ohm in the first post, but that is not correct. OEM value is 50 to 1000.
I also checked the wires. There are 5 wires that go directly from the cockpit to the sensors. 1 wire is the supply voltage that goes to all sensors. The other 4 wires are the "returning" ones from the fuel sensors to the cockpit. These wires are not open or short to plus. To make sure I made 5 new wires running through the trunk straight to the cockpit. This makes no difference. I strongly suspect the cluster itself. It will be tested this week by a rebuilding company. To be continued...
Instrument cluster has Component Protection and it has to be removed by a dealer. Problem is with the millage. It can't be adjusted (except if it is a brand new cluster). So donor car must be same as yours and millage on it should be close to your millage. Be aware that it is also recorded in the engine computer, so cheating is not advisable.
Where did you get the used cluster? My 2005 A8 keeps reporting a low or mid tank when it is full or has more gas than it reports. Same situation as I've last reported. Thing is, the car is locked out of the Instruments section (17). I heard on one obscure post that detaching the battery could reset it and re-enable my access to the car's readings. Anyone hear of this? All other modules work perfect and allow me access in VCDS just not the instruments anymore. Just times out and say's it's unreachable.