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DEF Locator is a very useful tool that provides information to consumers on where to buy DEF and how to use, store and handle it. To access, click the large button above this section. Thanks for visiting the API Diesel Exhaust Fluid Certification Program site!



Diesel exhaust fluid (also referred to as DEF or AUS 32) is injected into the diesel exhaust stream and the heat from the exhaust, the fluid, and a catalyst convert the NOx into the harmless gases.

Diesel exhaust fluid is a 32.5% solution of technically pure urea in purified water. For the SCR unit to operate properly, the DEF must meet an exacting purity standard. This standard, published by the International Organization for Standardization (ISO), is intended to ensure that the diesel exhaust fluid used in SCR-equipped cars and trucks provides the purity required by diesel engine manufacturers and the quality to preserve the emissions control system and the environment.

The API Diesel Exhaust Fluid (DEF) Certification Program is a voluntary program designed to certify and monitor the quality characteristics of diesel exhaust fluid intended for use in motor vehicles with diesel engines. Diesel exhaust fluid marketers that have demonstrated that their products satisfy the requirements of the most recent and applicable edition of ISO 22241, Diesel engines – NOx reduction agent AUS 32, may be licensed to display the API Diesel Exhaust Fluid Certification Mark. 

API Diesel Exhaust Fluid Certification has many benefits:  

  • Identifies for consumers diesel exhaust fluids that meet diesel engine manufacturer requirements.
  • Helps consumers find those fluids.
  • Gives consumers a choice of quality brands from which to choose.
  • Helps reduce Nitrogen Oxide emissions.
  • Provides rigorous monitoring of fluids in the marketplace.
  • Tests licensed fluids to ensure that they meet program requirements.

A complete list of licensees is available on our Licensee Directory.

Please download the Diesel Exhaust Fluid Program Overview for additional information and program requirements.

Please direct any questions or comments regarding licensing to

To login and manage a license, please visit

Proceed to the Applications & Fees section.