P0420 means your bank 1 converter is not doing its job good enough, most likely its starting to break apart. I had this code on my truck, well, p0430 for bank 2, and when I cut the pipe off after the converter I found a large chunk had broken off and was sitting sideways restricting much of the airflow. Not saying that happens in every case, but if you're going to get it tuned, its definitely worth considering gutting both converters. You can also tune out your EGR, Air pump if you have one, and Evap codes.
