Both the V12 and W12 are 'split' into 4 banks. Banks 1 and 2 are on the left side of the engine, as you look at it from the front of the car. Banks 3 and 4 are the right side. There are two lambda sensors per bank, labelled 1 and 2. 1 is the pre-cat, and 2 is the post-cat.
With that in mind, B3S1 'should' be sensor G285 (top left) in the attached image. The image is taken from the following SSP on the W12 engine:
SSP490 6.3l W12 FSI engine.pdf
If I was replacing it, I'd first disconnect G286, and go for a very short drive, and then scan the car again. If my numbering is correct, you'd then get an error code for B4S1 (G286). If that was the case, I'd then reconnect it, and replace G285.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
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2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds