The DEF systems on many modern diesel vehicles are a piece of crap that cause no end of problems. Shortly before I retired, the company I worked for bought some new PeterBuilts with the DEF injectors. There was a sensor on the exhaust that was supposed to tell the on board computer when the system needed to have the collected soot burned out and the DEF needed to be injected. An idiot light would come one to tell the driver the system was active. What was supposed happen was that the driver would allow the engine to idle while the system cleaned itself so that it would not overheat. Try that pulling a steep grade some place. One of our trucks overheated the turbo while the system was active and burned out the oil seal. Computer kept pumping more DEF because the sensors were indicating the system was still "dirty" from the oil going into the exhaust. Eventually the exhaust got hot enough to burn holes in it. Turbo, exhaust pipe, catalyst and DEF chamber were all toast. Peterbuilt fixed the dang thing under warranty, the bill for that would have been nearly $10k had the repair not been covered. Did not leave us with a warn fuzzy feeling about keeping any of these trucks after the leases expired. We were also getting a lot of error codes that were DEF/emission related, some of which would render the vehicle either inoperative or put it into "Limp" mode, not something you want in an over the road vehicle.
Dave