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What is DEF?


Diesel Exhaust Fluid (DEF) is a 32.5% aqueous solution of high-purity urea (ammonia) in demineralized water. DEF is utilized by the selective catalytic reduction (SCR) system to reduce the nitrogen oxide (NOx) emissions from diesel engines. DEF is a non-hazardous substance and is not a fuel or fuel additive. Vehicles equipped with an SCR system will have a separate reservoir with a separate filling cap for adding DEF. DEF should never be added directly to the diesel tank. Facts about DEF:

  • DEF is composed of 32.5% urea and 67.5% demineralized water
  • Consumption of DEF is estimated to be 2% of fuel consumption
  • DEF is non-toxic, non-polluting, and non-flammable
  • DEF may have a slight odor similar to ammonia
  • DEF will freeze at 12°F but is not harmed by freezing and is usable once thawed
  • Exposure to extreme heat and direct sunlight while in storage could degrade the DEF
  • Studies by engine manufacturers have shown a 5% increase in fuel economy using the SCR system
  • Maintenance, durability and performance of diesel vehicles is not affected by the SCR system