Fault code SPN 3226 FMI 10

[gemarsim]

Recently I bought a Volvo 2012 with ISX15 from a dealer in Chicago area. After 50 miles driving it this error popped out. Search of similar topic did not show any results.
Someone any experience with that? What I found browsing online is there should be two DEF sensors and one of them reads abnormal (value?)
What is the best solution? So far after 5000 miles the code is still active and the engine is running great. And it also takes DEF at normal rate.
Thanks to everyone with advice.