Staying On Top of Your DEF

As a diesel truck operator it is necessary to perform daily or pre-trip inspections of your rig to combat unexpected failures while on the road. All diesel engines 2010 and newer equipped with SCR require DEF, which must be checked during these inspections. The SCR (Selective Catalytic Reduction) is an advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. If DEF levels are not maintained, operation of these engines may adversely affect it's life span.

Driver filing DEF tank on truck


As required, the operator performing daily or pre-trip inspection should check the DEF level and top off tank as necessary to keep tank at least half full. Operating these engines without DEF may severely de-rate the engine. Upon inspection/filling, drivers should note if there is an unusual rate of DEF consumption. DEF consumption is generally 2% of the diesel fuel consumption. An excessively high or low consumption may indicate a leak or other problem. Only fill DEF tank with manufacturer approved DEF. The concentration (32.5% urea) and purity of the DEF is important for proper operation of the SCR system, which is why a diesel technician should check DEF Concentration at each scheduled service

At scheduled services, diesel techs must check for DEF system leaks to maintain proper operation of the SCR system and economy. A diesel tech must also be aware of manufacturer requirements and flush DEF tank, clean or replace filler, dosing or pump filters at recommended intervals. Performing these tasks at recommended (per manufacturer) intervals is necessary to maintain proper operation and longevity of the SCR system.


This information was provided in conjunction with the California Council on Diesel Education and Technology (CCDET), a non-profit cooperative group of California Community Colleges, the diesel industry, and government agencies, supporting training and educational programs for diesel mechanics.