WHAT IS DEF?
Diesel exhaust fluid (DEF) consists of 32.5 percent high-purity synthetic urea and 67.5 percent deionized water. DEF is slightly heavier than water at 9.08 lbs. per gallon (versus 8.34). Despite marketing names such as Adblue and Blue DEF, it’s a clear liquid with a slight smell of ammonia. DEF has a freezing point of 12 degrees Fahrenheit, but since both urea and water freeze and thaw at the same rate, it’s okay to use if frozen and thawed. Most trucks have a DEF heating system to keep the truck running in extremely cold weather.