S8s use 2 green sensors. One is in the V under the inlet manifold and the other is just behind the power steering pump. The 4-cylinders use one blue and one green.
I would say spark plug most likely, then coil pack, then excess carbon in that cylinder causing local hotspots and pre-ignition, then knock sensor. If it was failing one bank then sensor first, but since its one cylinder look at cylinder-specific things first
