Doc's Diesel DEF Diesel Exhaust Fluid: Doc's Diesel Exhaust Fluid (DEF) is one of those simple essentials that keeps modern diesel trucks out of trouble. When the DEF is clean and reliable, your equipment runs smoother, throws fewer after-treatment codes, and avoids power-cutting derates that cost time and money. Using high-quality DEF protects expensive SCR components and helps your truck stay compliant without constant shop visits.