Learn how to add DEF/diesel exhaust fluid to your 2019 Duramax by watching this DIY/how to video. In addition, you will learn where to purchase DEF, how much it cost, how many gallons your DEF tank holds, how to tell when it’s full, how to clean up any mess, and where to fill it up at. However, it is our duty to make sure that you understand how to add diesel exhaust fluid to your 2019 Chevy Duramax.