Have the ohtwo sensors on that bank tested first. P0421 is set when the precat and post cat ohtwo's have a too similar reading, indicating the cat is not doin' it's job. The precat sees a LOT of heat because of where it is, and may have failed, and may not have. I've seen this fixed with a 18mm thread plug fouling thingy from the Help section of the auto parts. It's just an internal and external threaded fitting that goes into the cat bung and then the cat goes into the fitting.

Not responsible for advice not taken...