I ran into a similar problem only my code was 460. I too also passed somg and my CEL never came on during it so that was good. My problem is that one of my Pre-Cats is busted.....but i don;t think thats it...if it were i'd notice a back up or unsmooth air flow...it seems to work fine....im thinknig of replaceing my O2 sensors and see if that fixes the problems( i can get them really cheap because i work for a parts wholeseller). Im also thinking of adding Sp Motorspots O2 enahncers wich are just wire harnest that attach to the o2 sensors that prevent false readings by changing the voltage ( good for custom exhaust)
The O2 enhancers might work for you. Ther 64 dollars at SP Motorsports for one pair ( and there is 4 on a v6) if your having no problems with passing smog this should cure you...but if you barly passed smog, I would not get them just yet...you could very well have a bad CAT, and that can get worse with time.
ROZ
