If you’ve noticed a tank with a blue cap on some of the newer diesel construction vehicles you rent, you may be wondering what it is. The answer: It’s a diesel exhaust fluid (DEF) tank. But what exactly is DEF, and why is it necessary?

Many newer diesel vehicles come with a selective catalytic reduction system (SCR) designed to control emissions before they are released into the air. The DEF tank holds DEF (a mix of water and a type of urea), which is sprayed into the catalyst of the of SCR. The catalyst breaks down polluting chemicals such as nitrogen oxides into harmless nitrogen and water.

In this video, United Rentals’ branch service manager Chris Weikle explains why DEF is important, where to buy it and how using it can benefit you.