I have an interesting issue with my 2005 A6 allroad C5 2.7t

I have an LPG kit fitted and mostly drive on LPG, I recently went in for some work on the LPG system and the mechanic noticed that my LPG settings were a little stange for this engine. He went to correct them and then realised that my petrol fuel trims were not moving. Which explains why the settings were strange for the LPG, they had been adjusted to take account to the fact that the fuel trims were not moving in petrol.

So knowing this I have started to investigate further. We replaced both lambda sensors and it made no change.

There are no engine fault codes and have never been any in my ownership (over a year).

I took a look with VCDS and checked lambda sensors and getting 0's across the board, which probably explains why the fuel trims arent moving, they are getting no data from the lambdas.

What is strange is that there are no fault codes?

Just looking for some advice as to where to start first.... Wiring issues? ECU fault? Something else related causing the lambdas to report nothing?

Welcome any suggestions.

Cheers