Does my turbo need replacing?

Thanks for all the help. I think I know someone with a fuel testing gauge so I'll talk with him.
As far as pulling the DPF....I have no experience with the emissions stuff. We get rid of most of that stuff because of the amount of idle time in oilfield work. But I'll watch a couple videos and see if its within my mechanical abilities. Lol

 

One thing the truck does once in awhile...only when I'm going downhill for some reason...it'll throw a Nox sensor code and tell me it'll derate in 4hrs. Then the code will go inactive. But it says "SCR fault" on the screen while the check engine light is on. Never derates....and the check engine light will go away after a few key on/offs.
I did notice a build up of dried DEF fluid and tightened the clamp on (I think the DOC?). The decomp tube had been changed shortly before I bought it.
Again, when it comes to after treatment...I'm in the dark.
Not sure if maybe there's a problem with the SCR but the only readable code on the screen (and in InSite) that I've ever gotten is the NOx code.
I probably should have mentioned this in the original post...but it's really intermittent and only happens going down a hill with the Jake on.