I have a 2001 A6 Quattro 4.2 V8 and my catalytic converters rusted at the flex pipe. I went and ordered 2 converters since this car has two of them and I want to order the O2 sensors also but I dont know how many for each pipe cause when I'm looking at parts it says that one is for upstream and another is for downstream or one sensor can be either upstream or downstream. If some can plz help me out with what ones I need and how many for each pipe that would be great.