If you've ever driven a diesel truck or car, you know how powerful and efficient these vehicles can be. Unfortunately, this strength comes at a cost because it's difficult to maintain the DEF filter in diesel cars. If your DEF filter isn't working correctly, several warning signs indicate you need to replace it as soon as possible. DEF is a urea-based solution that reduces NOx emissions from diesel engines. It is added to the fuel tank of diesel engines, made up of urea, deionized water, and antifreeze to prevent freezing at a sub-freezing temperature which is between 30-40 degrees Fahrenheit - Detroit ONE BOX full Cleaning Service . DEF is a crucial component in keeping your vehicle operational, just like any other type of engine fluid. Before the exhaust comes out of the tailpipe, a water-urea mixture is injected into it to cause a chemical reaction. This reaction converts harmful pollutants into clean air and keeps them from entering the atmosphere. A DEF filter ...