Same Story DIfferent Truck

[Rawze]

25k and 60 mph, I would be breaking 9 mpg everywhere out there(or 8 in a newer engine any ways). You have something seriously wrong.

Emissions problems are ALWAYS ALWAYS the RESULT of engine and/or sensor problems. At 550k i doubt you will be able to keep the emissions systems happy any more. Pull the DPF and DOC and take pictures of their faces, then go have them baked for a few hundred bucks. Post the pics on the forum for some opinions.

Those engines are notorious for NOX sensors to go bad without throwing codes too. Howe old are they? They typically live about 200-300k miles and then read wrong, causing a lot of ghost problems.

When is the last time the decomp tube was inspected and the DEF injector cleaned, filter for it changed>?