Def problem keep getting Knox effecent code

[Crownvicryder]

Hey guys I have an 2013 cascadia with DD15 and I keep getting Knox effecent code now I've been chasing this problem for last year and paid out a lot of money to try and fix almost everything under the Def and dpf I've changed with the exception of the big 3 filters but dealer says it has its own code so wouldn't be that. I've replaced def filter , pump ,tank, def pressure sensor, def doser, Knox censor , cleaned the lines thinking they were contaminated and nothing fixed the issue. Now at best it will be good for a week or 2 after dealership says it's fixed then it will derate usually when I'm pulling an 38000 lbs load or more up and down hills . It really dose it when eng has too work and it's hot outside. At this point I'm thinking diesel tuning is only fix I can't keep paying huge repair bills every few weeks . I need help guys please.