Frequently Asked Questions About Diesel Exhaust Fluid (DEF)

What is DEF? DEF is Diesel Exhaust Fluid. 

What purpose does DEF serve?  DEF is used in Selective Catalytic Reduction systems to change NOx in diesel exhaust to Nitrogen and water.

How often will I have to refill my DEF tank?  That depends upon the size of your DEF tank and the dosing rate chosen by your automobile/truck manufacturer. DEF tanks range in size from approximately 6 gallons to 30 gallons.

Will my truck sustain any damage if I do not refill the DEF tank?  No.  

What will occur with my truck if I do not refill the DEF tank?  Performance degradation to be followed by an inability to start your vehicle will occur if you do not fill the tank.

How can I know what to buy?  High quality DEF will be produced and distributed by companies that have demonstrated their domestic expertise in production and distribution and will bear the API Certified symbol. This designation implies that the fluid meets the stringent product quality standards established by ISO 22241.  

Why is it necessary to use DEF now?  Diesel engines purchased after January 1, 2010 require the use of DEF to meet the EPA’10 tailpipe standards.

What are the benefits of using DEF?  Diesel engines equipped with SCR using DEF not only meet the 2010 tailpipe standard, they also deliver significant fuel economy.

Are there any storage considerations/limitations for DEF?  Yes, since DEF can degrade over long periods of time (>1 year), and at temperatures over 90˚F, long term storage should be limited, if possible.

Will my vehicle sustain any damage if I accidentally spill the DEF on the engine or other surface?  DEF can be mildly corrosive to some materials, particularly aluminum, copper and other alloys. It should be washed off with water.

What factors degrade the DEF?   Product should be stored in temperatures between 12º F and 86º F.

Additional questions about DEF?   Contact us at 1.800.950.3835 or


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