To finally put to rest the issue that sparked this thread almost 5 years ago...the DTC P0420 was in fact related, in my case, to a failed precat on the right side. Unable to find any OEM precats on the market, this past weekend I had it replaced with a Miller Cat part #74004 which I purchased on ebay for $150. This particular part has an EO# making it legal for installation on Gen 2.5s in California. I had the new precat installed at Mike's Performance Exhaust and Radiators in LA for another $150. I had an excellent experience at Mike's and would highly recommend his shop to anyone. Then I cleared the check engine light, drove around to ready the monitors, took the truck to get smog tested and passed!