So, some quick history. Had the car for 3 months now, '05 w/64K miles, it's bone stock, about 3 weeks ago driving on the freeway, traffic slowed, downshifted from 6th to 3rd, popped the SES light with code p0136, had it scanned and cleared thinking it was an erroneous occurrence.

Fast forward to tonight, driving home from work on the freeway, cruising in 6th at about 70 with the cc on/set, and the light comes on again with the above codes in that order (2270, 2270, 0136).

I've read a lot of posts but none really seem to hit the nail on the head as to the exact cause? Am I looking at an o2 sensor? Did I fubar a gasket somewhere in the exhaust setup? I've seen people post to test for an exhaust leak, but how do you do that exactly? Crawl under and try and smell for it?

The first time it came on...it got me thinking about exhaust systems, and I'm already considering a catback from Invidia. So if it's an o2 issue, I'm going to do a few other things while it's all apart (grease shifter cables/box, maybe the rear sway, etc). Does a header really make all that much of a difference on our cars? Eventually I'll get to a CAI as well, but trying to plan repairs/upgrades in groups/stages.

Any input/help would be appreciated, as always.

Thanks!