On my A6, I saw 2 o2 sensors right towards the rear of the engine on the exhaust pipe. The scary thing is, the bentley mentioned 4 o2 sensors. I am not sure what cars it applies to. But it definitely talked about "before and after cat measurements". On the source of o2 sensors, are all 3 wire Bosch o2 sensors the same? My Dodge uses Bosch o2 sensors, for a grand price of $40 each. Xudong