I have an 08 TBSS with 63,000 miles. The check engine light has been coming on for the past month and a half. We've taken it to the local Chevy dealer, 2 other mechanics and a performance exhaust place. My husband has a code reader, and the code always comes up P0420. The bank 1 cat has been changed to a Magnaflow cat. Both of the bank 1 sensors have been replaced. A vacuum hose on bank 1 has also been changed. The longest the check engine light has remained off was 3 days. It was worked on this past Wednesday. The check engine light came back on this afternoon. Has anyone had a similar problem?