If you own or operate a modern heavy-duty diesel truck, you’ve almost certainly added Diesel Exhaust Fluid (DEF).
But despite using it regularly, many operators still have questions:
- What exactly is DEF?
- Does it go in the fuel tank?
- Why do diesel engines need it?
- How often should you refill it?
- Can you drive without it?
- What happens if it freezes?
- What if you accidentally put DEF in your diesel tank?
The good news is that DEF isn’t nearly as complicated as many people think.
Although it never enters the engine itself, Diesel Exhaust Fluid plays a critical role in helping today’s diesel engines meet emissions standards while maintaining performance and fuel efficiency.
Whether you operate a single truck or manage an entire fleet, understanding how DEF works can help you avoid expensive repairs, unnecessary downtime, and emissions-related problems.
Quick Answer
Diesel Exhaust Fluid (DEF) is a mixture of 32.5% high-purity urea and 67.5% deionized water. It is injected into the exhaust system—not the engine—to help convert harmful nitrogen oxide (NOx) emissions into harmless nitrogen and water vapor.
Continue reading What Is Diesel Exhaust Fluid (DEF)? Everything Heavy-Duty Truck Owners Should Know Diesel Exhaust Fluid (DEF) is a mixture of 32.5% high-purity urea and 67.5% deionized water. It is injected into the exhaust system—not the engine—to help convert harmful nitrogen oxide (NOx) emissions into harmless nitrogen and water vapor.
