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DEF stands for diesel exhaust fluid. It is a fluid used by diesel vehicles and equipment with selective catalytic reduction (SCR) emissions systems—not diesel fuel or an engine additive. DEF helps the SCR system reduce nitrogen oxides in the exhaust.
What is DEF made of?
DEF is a 32.5% solution of technically pure urea in purified water, according to the American Petroleum Institute (API). In the ISO context, it is also called AUS 32; AdBlue is another name used for the fluid in some markets.
What does DEF do in a diesel vehicle?
An SCR system injects DEF into the exhaust stream. With exhaust heat and a catalyst, it helps convert nitrogen oxides (NOx) into nitrogen and water vapor. DEF goes in the vehicle’s dedicated DEF tank, not the diesel tank. Check the owner’s manual for the correct fill point and procedure.
Which vehicles use DEF?
DEF is used by diesel vehicles and equipment fitted with SCR; not every diesel vehicle has that system. The U.S. Environmental Protection Agency says nearly all on-road diesel trucks and many types of nonroad equipment have used DEF as part of SCR systems since 2010. That statement is not a precise market-share estimate and does not establish that every diesel model uses DEF. Consult the vehicle manual and dashboard warnings to determine what your vehicle requires.
#1 Best Overall
- One gallon of is used approximately every 300 miles
- A truck with a 20 gallon DEF tank can go up to 6000 miles between fill-ups
- When used in an SCR system will reduce the levels of NOx emissions of those engines
- Fit type: Universal
How to choose and store DEF
Fluid quality matters because off-spec or low-quality DEF can damage emissions-system components. API warns of risks including catalyst poisoning, deposits, and clogged injectors or filters. A stated urea concentration alone does not prove that a product meets all required purity characteristics.
Quick Recap
Best Value
- ENGINEERED: For use in Selective Catalytic Reduction (SCR) systems on diesel engines
- EFFICIENCY: BlueDEF diesel exhaust fluid is key to helping improve fuel economy by up to five percent
- PROTECTION: Diesel exhaust fluid is non-toxic and safe to handle
- QUALITY: Stable, colorless and odorless, and also meets ISO Standards 22241 for purity and composition
- EFFORTLESS: Easily Inject the fluid into your exhaust stream with a specialized dosing module
Rank #4
- Blue Def DEF002-2PK Diesel Exhaust Fluid, 2.5 Gallon, 2 Pack
Rank #3
- Meets GM Original Equipment specifications to be approved for use in GM vehicles as well as in non-GM diesel vehicles that have an SCR (Selective Catalytic Reduction) system to meet regulated NOx emissions (Based on EPA certification testing. Your actual emissions will vary based on several factors including temperature, terrain, and driving conditions).
- High quality formula is made from high purity urea and deionized water.
- Meets API regulations and ISO 22241 standard for purity and concentration
- Nozzles sold separately
Rank #2
- One gallon of is used approximately every 300 miles
- A truck with a 20 gallon DEF tank can go up to 6000 miles between fill-ups
- When used in an SCR system will reduce the levels of NOx emissions of those engines
- Look for packaging that clearly states the fluid meets applicable ISO 22241 requirements. ISO 22241 covers AUS 32 quality, handling, transport, storage, refilling interfaces, and test methods; see the ISO 22241 series overview.
- The API certification mark is another useful cue for fluid licensed under API’s voluntary certification program. API certification is not mandatory and is not the only way to establish compliance. See API consumer guidance and its purchasing and handling advice.
- Follow the storage instructions on the container and in the vehicle manual. API advises against storing DEF for too long in a vehicle when the storage area routinely exceeds the temperatures recommended on the label.
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