ive been told to replace my o2's 2 times (lean 02 code) and i havent once done it. it took me a while to find the problem but what it was is i had a leak in my exhaust manifold gasket off cylinder 7 so it was pulling fresh air into the exhaust before the o2 sensor making it read lean and the truck started dumping in alot of fuel and making it smell like it was running rich so if u havent do a quick check for a leak some where, if ur trucks loud u might not have hear it