It should be disassembled and removed. I'd be looking for evidence of def deposits, discoloration, and erosion of the catalyst substrate. A visual inspection doesn't show everything though.
If the mass airflow sensor and scr inlet temp sensor readings are plausible, def concentration and cleanliness are good, NOx sensors and def injector are new, that pretty much leaves the SCR catalyst as the prime suspect, in my experience.
I got another question, lately we have been having a bunch trucks, mostly 2018 268 conventionals. These 2 codes kill us on come backs P04DB crankcase breather code, and P0101 air flow sensor code. We have either replaced and or cleaned egr cooler and egr valve on several, some haven't returned others are coming back. A couple of the comebacks are now throwing P226c VNT slow response. The air flow code is something else, following troubleshooting. You got any advice?