I have the same issue, the code reader reader from an Autozone reads out code P2201. I plugged in my MaxiDAS 808 and it is reading errors in the upstream Nox sensor.

Which is the upstream and which is the downstream Nox sensor? There is one in between the DPF and the SCR and another between the SCR and the muffler.

 

I wanted to provide an update on my diagnosis of the P2201 Nox sensor fault code. I took it to my local dealer who wanted to replace both the upstream and downstream Nox sensor for $1600.

With my MaxiDAS 808 ODBII computer, I read the fault code as a plausibility error in the upstream Nox sensor only so I didn't believe the dealers remedy. I ordered the Nox sensor mail order for $320 and replaced it. While I was under the GL, I pulled off the DEF injector from the SCR and noted the opening was almost completely plugged with crystalized DEF. A few minutes with a screwdriver and vacuum cleaner and the DEF injector opening was clear.

There is a parametric calibration routine that needs to be performed after replacing a NOx sensor. I ran that routine with the Autel MaxiDAS 808 and it reported success.

Everything seems to be running well 3 months after the replacement.