"Peterbilt 348 w/paccar px-9 (2021)

I have a Peterbilt with a px-9 engine which is what im assuming just the same as a L9 cummins with code
spn 3521 fmi 18. im sure it is pointing out to be an issue with the def system. i have tested the def with a refractometer and it shows it is fine and did do a change in the def filter as well. has anybody else have an issue similar that can be of some help. i cleared the code but noticed it has returned..

 

I was scared it would be that as i have already changed one in the past with our other trucks and they do not run cheap at all. considering it is such an extremely expensive part. i would of thought they would atleast fix the issue if its so common a part that goes bad..considering the costs..