i have noticed for a while now that the back of my car is always coverd in alot of carbon. i did a complete tune up and changed the fuel reg. and filter. All that money wasted because the prob wasnt fixed. i went and spent the 40 bucks to have the car checked out on a scanner and my mechanic told me that my o2 sensors are fine and that my cat is shot. any one have any sugestions or maybe know somthing they dont?? i was looking into a high flow cat converter because i want the prob to stop. any help would be highly appreciated.