Currently getting P0420 (Cat below threshold bank 1) and C1201 (Engine control system malfunction)
Car drives fine, though the trac light is on, as well as the CEL.
Recently replaced the starter, drove it for about 2 weeks before this issue cropped up. I cleared the codes yesterday, it takes 30-40 mins before they reappear.
Bank 1 is driver's side I believe, I didn't touch the upstream 02 sensor at all, and only removed the downstream one while pulling the exhaust.
Anything else it could be? Techstream doesn't seem to differentiate between upstream or downstream, just buy 2 sensors and hope for the best?

 

Techstream definitely differentiates between B1S1 and B1S2, B2S1 and B2S2. The S1s are the wideband sensors in the headers. The S2s are the narrowband downstream of the primary catalyst. Since the S1s have nothing to do with catalyst efficiency, this code is for the S2 narrowband sensors only. Did you replace the gasket between the header and the midpipe? If that is leaking even a little bit, you'll get the catalyst efficiency codes. That's where I'd start if it were mine.

 

Given that the CEL doesn't immediately trigger, a small leak was my guess as well. Using a Joe Z exhaust, I'll check at the header/midpipe junction

 

Super fun discovery, decided to pop my head underneath on cold start and found this. Pushed out two tiny drops of water from the welds, very surprised as there's no corrosion on the car and and I live in the South.
Gasket junction appeared to be solid and I didn't feel any air leaks.
I'm leaning towards an RR racing tune that deletes the CEL and call it a day. Having just done the starter I'm not eager to do the job again

 

Nice find. Not good news, but at least you know where the issue originates.