MP4.2+6.0

Unless you think cat is bad (usaully only at high miles and bad gas or big oil burning issues), a post cat fuel trim lean is likely the PRE-cat O2 sensor starting to act up. But worst case, change both for that bank. Weirdly on an S8/4.0T, the O2 sensor connectors for pre- and post cat on a given side of motor are NOT in the same location. Bank 1 pre is grouped with Bank 2 post, and Bark 2 pre- with Bank 1 post. BTDT. When you look at how the cats and exhaust headers are laid out and how they twist, you will see why. One set of O2 connectors is easy to reach in motor area. Other set down by the passenger side front CV drive shaft is a total bitch--hard to reach in general, need to pull a key metal shield around inner CV joint, and especially both how the connectors unclip and re clip from mounting body point are very tedious.

Bank 2 Ia driver's side for US/Continetal Europe; hopefully the pre- is the one on that side with the connector easy access under hood [I on't recall]. OES on O2 sensors is typically Bosch, and way cheaper than dealer ones.

A8L_New_England

.
From what I gather, some fuel trim lean issues are due to a defective HPFP.
.

MP4.2+6.0

If it helps, my 2015 S8 would code typically just driving down the highway at moderate and steady speed after 15+ minutes.I cleared it a few times to see if it would come back consistently, which it did. I had some sense it might have been right at the point the 8-4 part cylinder run mode (COD) cut in, but by then I had ordered sensors anyway. I did all since I was in there and Bosch's weren't a lot of money once I was doing the labor. But from experience now if pre- cat sensor for that bank is in the easy to get connector set under hood, I would start there and surgically. If the post cat one for that side is the one with the under hood connector, you can try that. But counter intuitively perhaps, many post cat code errors tend to signal what is actually a pre cat sensor issue. The catalytic reaction going in the converter is the hidden in between thing affecting the readings. Said another way, post cat sensors rarely fail and are simpler designs and function than the front ones. So don't be surprised if code returns until front sensor gets changed.