Throwing code P2A00 after installing stock cats

ArtVandaleigh

I've been having this code over the past month after returning my car to mostly stock. A little history first, previously I was running the following setup with no codes for 6K miles:

Berk HFC
Nismo Catback/FI Catback (FI for last 500 miles on Berk HFC)
HKS filters

Went back to all stock except for FI catback a month ago and immediately started getting the P2A00 error after about 50 miles driving, i reset the code and again it appeared. This error is described on my code reader as "02 bank 1 sensor 1 circuit range/performance". Now my initial thought on this after some research was, possibly a leak of some sort... but all gaskets were replaced new and we triple checked any leaks.. none, same with the intake. The error is supposed to relate to the sensor in the passenger header manifold as well, and this was never touched, or bumped by accident. It's also in a major PITA spot to get too.

One thing to note though, is when i was running completely stock a year ago except for a Nismo Catback, I added new HKS filters and got the same P2A00 code I'm receiving now. This went away though after I replaced them with the stock filters. Currently I am using the stock filters, the only thing that has changed would be the FI catback from my original setup. Would the FI catback considerably change the A/F ratio to throw a code? Besides cleaning the MAF sensors (which only have 18k on them) or replacing the O2 sensor, I do not know how to proceed on this. I highly doubt it's the sensor itself, just the lean/rich tolerances. Any thoughts on this? I dont want to take this to the dealer and start racking up charges for diagnostics, is there an easier way i can determine if my A/F ratio is out of spec? Thanks for all input.