DEF stands for diesel exhaust fluid, ... The system works like this: ... Here's when DEF became mandatory for semi-trucks and some information about the other vehicles that must use it.

One of the main updates to the DEF has major implications for cold-weather operators. DEF is about two-thirds water by volume, and water freezes up and clogs the system in cold weather.

Diesel Exhaust Fluid is a critical component in a diesel truck's fuel system. Here's what it does and how much you can expect a truck to use.