The Basics of Diesel Exhaust Fluid (DEF)

Diesel Exhaust Fluid (DEF) is an important component of today’s diesel technology. Its use in Selective Catalytic Reduction (SCR) leads to cleaner exhaust and lessens environmental pollution. Here’s everything you need to know about how SCR works and how to properly maintain your DEF.


SCR is technology used to process emissions so that diesel engines will meet emissions regulations. SCR systems are found on 2010 and later model trucks, and on many diesel pickups and SUVs.

A part of SCR involves the injection of DEF into the engine exhaust. The DEF chemically reacts with the nitrogen oxide found in the engine exhaust before it is released through the tailpipe, ultimately reducing the nitrogen oxide in the exhaust by up to 90% by breaking it down into harmless hydrogen and water.


DEF is a non-hazardous solution of 32.5% urea and 67.5% de-ionized water that is consumed during SCR. DEF is not a fuel additive and never comes into contact with diesel; it is stored in a separate tank, typically identified by a blue filler cap.

DEF is stored in a tank on board the vehicle and is automatically injected into the exhaust stream by a metering system at a rate that is dependent on the specific after-treatment system, but which is typically 2–6% of the diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimizes the tank’s size.

DEF is colorless, non-toxic, and non-flammable, and small amounts can be rinsed with water if spilled. That being said, although it is not classified as a harmful material, it is recommended to be handled with caution because it is corrosive to plant life and most metals, which can make storage a challenge.


Even a small amount of trace elements can contaminate an entire tank of DEF and ultimately cause damage to your SCR systems, which are sensitive to chemical impurities and particulate matter. In addition to damaging the catalyst of your SCR system, contaminated DEF may also damage other components of your machinery. A system using contaminated DEF will consume more of the fluid and be less effective at reducing emissions from the exhaust, ultimately hurting your bottom line.

Because of the corrosive nature of the fluid, it is very important that it is properly stored in certain materials such as stainless steel and polyethylene. Improper storage can lead to contamination from outside sources. If the fluid is stored in the wrong type of storage system, the fluid can corrode the metal, which will contaminate the fluid and may ultimately lead to a major spill.


Follow the four best practices below to ensure that your DEF stays pure and allows your SCR to work efficiently:

  1. Purchase Pure Fluid: Make sure that the DEF you purchase meets the ISO standard by looking for the API diesel exhaust fluid certification mark.
  2. Store Properly: Use a container made for DEF. It should be constructed with standard-approved materials such as stainless steel and polyethylene that will resist corrosion, and it should be sterilized and sealed properly.
  3. Maintain and Clean Closed Fluid Path: The standard allows for the use of an open path system, but they must be sterilized between each use, which isn’t practical in off-road environments. Using a closed system is recommended in these cases.
  4. Keep the Workspace Clean: Dirt and debris near the DEF fill port can lead to contamination of the fluid. By keeping a clean workspace, you will reduce the risk of contamination.

Please contact us to learn more about how our full range of DEF storage solutions, including the Blue Pro and the Abbi, can help you meet your operational goals.


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