What is DEF (Diesel Exhaust Fluid)

What is DEF

DEF (Diesel Exhaust Fluid), also known as AUS 32, is a high purity chemical solution, comprised of 32.5% urea and 67.5% de-ionized water. Urea is produced by the synthesis of ammonia and CO2 (carbon dioxide). DEF is produced from Automotive grade Urea, a particularly pure grade of Urea, with low levels of metals, minerals, and other substances potentially harmful for the SCR system and catalyst.

It is important to only buy products named DEF, manufactured according to the ISO 22241 standard. There are many products available, such as “Urea solution” or “Aqueous Urea Solution” that do not meet the ISO standard and specification.

SCR systems with which DEF is used are very sensitive to potential chemical impurities that can contaminate the urea solution. Even small amounts of impurities can damage the SCR catalyst. Using a quality product will avoid the high costs of having to replace a damaged catalyst.

DEF production is regulated by the American Petroleum Institute (API). API offers further information on its website www.apidef.org under the theme “DEF” with an actual list of certified producers. Only buy DEF from a company that appears in the API DEF directory, or from a distributor of those brands.

Warning: most vehicle engine manufacturers do not accept warranty requests in case a low quality DEF has been used.

Always test DEF

To be sure that the product you bought is in line with the specifications you should run a full, very expensive, chemical analysis.

A basic test that helps to avoid most bad products is the “Urea Concentration”: the result of this test must be between 31,8% and 33,2%.

Below find the best online deal to buy a cheap refractometer and do the tests yourself.



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