Hello... I just picked up a 99 S.As I wasdriving it, I noticed a sulfur, rotten egg smell. I did a long search and come up with a couple of things. It could be the CAT, O2 sensors or KNock causing the smell? I have no Check engine light on.. and I noticed that the car has three cats... How Do i know which one to replace. I was going to put the car on a snap-on scanner I have to see if anythingcomes up...Any help would be great...

Also the T-tiimg needs to be replaced at what mileage? 100k.?