Sensor bank 2, aka downstream, was replaced. Cat isn't factory. Don't have $600 for that. Cat broke free from the resonator for 2 weeks before I could fix it. Goddamn that was a loud drive. Haha!
I can't weld for crap, so I replaced both ends. Good thing I did because I discovered a crack half way to the muffler I didn't know was there. Along with the flex pipe cracked behind the exhaust manifold. New exhaust has no leaks. So P0420 code doesn't make sense.
Bank 1 (upstream) hasn't been replaced as it came out of the old cat just fine. I had to replace bank 2 because it was basically seized into the old cat and didn't budge. Connections are clean and secured. Made sure of it before I installed the new cat.
What is maddening is it only sets the code at idle, 3 times now at a red light, halfway through the drive cycle.