1999 XLT 4.6L Triton V8 199,280 miles

I think I have a bad bank 1 catalytic converter. It all started with a P0420 code. Misfire plug 3 code soon after. I cleared the codes, changed the plugs and changed O2 sensors up and down stream; no new error codes yet. It’s running decently but the live data has me pretty sure I will be replacing the cat. At operating temp, my downstream O2 sensor is oscillating pretty wildly on bank 1. Bank 2 downstream is smooth.

I think this all stems from when I had to replace the intake manifold gasket about 8 months ago. It leaked all in my spark plug wells 1-4 and the engine valley. I replaced the gasket, kept the solid metal intake manifold (they make a plastic one?!) and it has been leak free. But I think that nasty stuff fouled my catalytic converter on that side. I think it’s finally giving up the ghost.

I already bought the direct fit cat for right side (y-pipe), but just thought I would check if I’m missing something.