Definition
Diesel Exhaust Fluid, a urea-based fluid used by selective catalytic reduction systems to reduce certain emissions.
Practical example
Example: The driver checks the DEF level before a long route and uses the correct fill port.
Why it matters
Running low or contaminating the system can cause warnings and reduced engine performance.
DEF in CDL training
CDL students should be able to recognize this term, explain it in plain language, and connect it to safe vehicle operation, inspection, licensing, or freight work as applicable. Review the term in context and practice using it during classroom study and hands-on training.
Frequently asked questions
What does DEF mean in trucking?
Diesel Exhaust Fluid, a urea-based fluid used by selective catalytic reduction systems to reduce certain emissions.
Why should a commercial driver understand DEF?
Running low or contaminating the system can cause warnings and reduced engine performance.
This glossary is educational. Always follow current federal and state rules, the vehicle manufacturer’s instructions, and your motor carrier’s procedures.