In the September 2014 issue of Diesel World we talked about diesel exhaust fluid (DEF), which is used as an exhaust treatment in modern diesel emissions systems to reduce the levels of nitrogen oxide (NOx) in the exhaust. These systems exist on most modern diesel pickups, diesel cars, semi-trucks and farm equipment. Used properly, DEF is a good thing—but used improperly, it can cause serious (and seriously expensive) problems.
https://www.dieselworldmag.com/diesel-technology/when-def-is-death/