If I'm not mistaken, the only plugged up O2 bung in the stock exhaust is on the 90 degree bend connecting the s-pipe to the stock exhaust (the part that the Corksport exhaust bolt to, our stock j-pipe is two pieces). It is located on the side underneath the piece's heat shield. I am thinking that we will have to use that plug, since that 90 degree piece will be replaced with our exhaust. Also, I am pretty sure you will have to plug up the first O2 bung (before the cat) on the new exhaust. Otherwise, what is the point of having the cat? You would still get a CEL!