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Diesel Exhaust Fluid (DEF)

What is DEF?

DEF is Diesel Exhaust Fluid.

What purpose does DEF serve?

DEF is used in Selective Catalytic Reduction (SCR) systems to change the Oxides of Nitrogen (NOx) emissions in diesel exhaust to Nitrogen and water. Nitrogen makes up about 78% of the air we breathe every day.

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. Dosing rate is approximately 2% - 3% of diesel usage. DEF tanks range in size from approximately 6 gallons to 30 gallons.

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 ISO22241. 

What are the benefits of using DEF?

Diesel engines equipped with SCR using DEF not only meet the 2010 tailpipe standard, but they also deliver significant fuel economy.

Is DEF usage required by law?

Yes, in diesel engines produced after January 1, 2010, DEF is required for emissions after-treatment.

Are there legal penalties for not using DEF?


If I buy extra DEF, can I store it in my truck?

Yes, but avoid direct exposure to sunlight so that the product does not get hot.

Are there any storage considerations/limitations for DEF?

Yes, since DEF can degrade over long periods of time (>1 year), and at temperatures over 86˚F, long term storage should be limited, if possible.

What if my bottle of DEF is several years old and still sealed? Can I still use it?

Yes, the reagent in sealed bottles of TerraCair that are less than 2 years old are probably just as effective even though, upon opening the bottle, you may notice a slightly greater ammonia odor.

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.

Why is it necessary to use DEF now?

Diesel engines purchased after January 1, 2010, require the use of DEF to meet the EPA 2010 tailpipe standards.

What factors degrade DEF?

Product should be stored in temperatures between 12º F and 86º F.

Should my DEF tank be emptied if my truck is placed in storage?

Not required. Vehicle on-board controls are designed to store and handle DEF.

What if DEF gets on my hands or in my eyes?

According to OSHA, DEF is mildly irritating to the eyes and skin. If the liquid should contact either, the DEF should be flushed out/washed off with plenty of clean water. For more health and safety information, refer to the TerraCair MSDS.

What does TerraCair look like?

TerraCair® is clear like water.

What does DEF smell like?

The liquid can have a slight ammonia smell.