Diesel Exhaust Fluid, or DEF is becoming more and more important to commercial vehicle operators as environmental regulations become more and more strict.  But what exactly is DEF?  In short, DEF is a solution made of urea and deionized water used to lower the concentration of Nitrous Oxides (NOx) in emission from diesel engines. 

DEF must meet ISO exacting purity standards.  It will also likely be certified by the API, although this program is voluntary.  DEF should contain 32.5 % urea and 67.5% deionized water according to ISO 22241 standards.   Because of its composition, DEF is susceptible to freezing (it generally freezes at -11 C).

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