plenum has been replace less the 2000 miles ago. I think its the stock cat. No check engain light.

 

Positive cranckase ventilation. Clean and inspect it. It helps a lot.

 

CEL would only come on if the post-cat 02 gets pissed off or there is a misfire.

A clogging cat would not set a CEL unless it was causing a misfire.

Just yank the damn cat. I got lucky with mine, it was kind of starting to clog, then it got busted up by my exhaust guy.

 

Just "unscrew" the pre cat o2 and see if it runs any better. No need to drill holes.