I have a P0420 code that is not going away. Been going through a lot of threads on replacement cats and it seems aftermarket fitment is hit or miss. I'm debating keeping OEM, spending the $$$ and ensuring good fitment and function vs. aftermarket. I want to keep the system stock (no removing secondary cats, long tube headers, etc.). A few questions... P0420 is "bank 1". That is the passenger side, right? Should I also replace sensors (oxygen sensor? any more?) at the same time? Rockauto has 3 groups. Among big names, "OE fitment" has Walker and Nissan OEM. "Improved design" has eastern catalytic. And "aftermarket design" also has eastern catalytic. Anyone know what the difference is between the 3 groups? For those who have had this work done at exhaust shop, how much did it cost you? Ultimately, I just want something that is gonna bolt in without having to shave things down or pull on something hard because a weld is off by a few degrees. And I want it to last. Am I stuck with OEM then?