My OBC always reports 999km range irrespective of the actual amount of fuel in the tank. All other OBC functions work correctly as does the fuel gauge itself. Anyone have any ideas as to what the source of the problem might be?

 

It looks like a module called the central informer (gotta love that name!) feeds fuel level and speed info to the OBC within the tach and also provides the same info to the speedo and fuel gauge. If your speedo and fuel gauge are working OK then I would suspect the OBC itself. You can try disconnecting the battery for a few days to see if that has any effect or swap with a known good one.