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Diesel Exhaust Fluid (DEF):
A Primer


If your diesel engine was manufactured in 2010 or later, then you’re familiar with filling the diesel exhaust fluid (DEF) tank. But what exactly is DEF for diesel engines and does it add any extra maintenance concerns?

What Is DEF?

Diesel exhaust fluid is a liquid solution of 32.5% urea and 67.5% deionized water that gets injected into the exhaust stream of diesel vehicles to improve emissions.

Specifically, DEF converts harmful NOx gases (nitrogen oxide) into nitrogen and water. DEF looks like water but has a smell of ammonia, similar to household cleaners. DEF is not mixed with fuel nor is it added to your fuel tank; it is stored in its own tank and injected into the exhaust as you drive.

Why Do Commercial Diesel Trucks Need DEF?

In 2010, the Environmental Protection Agency (EPA) passed emissions standards that required diesel vehicles to reduce the amount of particulate matter and NOx they emitted.

To meet these regulations, the manufacturers designed Selective Catalytic Reduction (SCR) technology, an advanced catalyst system that cuts down on emissions. Due to these regulations, diesels sold in 2010 or after come equipped with SCR tech and a DEF tank. The DEF must be refilled regularly.

What If My Diesel Runs Out of DEF?

Modern diesels are programmed specifically for the use of DEF. Running out of diesel exhaust fluid can cause reduced performance, including cutting power, throwing a check engine light and forcing the vehicle to reduce speed.

Common DEF Problems to Watch For

High-quality DEF is essential for modern diesels to run properly. Problems with the DEF system or the fluid itself can lead to increased DEF consumption, reduced effectiveness and various malfunctions. Some of the things to look out for include:

  • Mixing DEF with diesel fuel: The DEF tank is often right next to the fuel tank. The DEF tank cap is blue and has a smaller fill hole. But when in a rush, drivers sometimes fill their regular diesel tank with DEF, and that can cause major issues. The tank must be drained and flushed, along with the entire fuel system.
  • Crystallization: Too much DEF in the system can lead to crystalline deposits in the exhaust or injector nozzle. Crystallization can also be caused by a stuck-open injector nozzle, plugged DEF lines or DEF pump problems.
  • Contamination: DEF is highly sensitive to contamination. Problems occur if dirt, rust or scale gets into the fluid and damages the DEF pump. DEF is also easily contaminated by other fluids like coolant or diesel, which can damage SCR systems and commercial diesel engines.
  • Additives in cold weather: Don’t be tempted to add anything to your DEF tank other than DEF. Additives, including those that are supposed to help in cold weather, throw off the chemistry of the fluid and cause system damage.

Bona Bros: Your DEF & Diesel Truck Body Shop

You can purchase diesel exhaust fluid at either of our two Twin Cities Bona Bros locations. And, if you believe there’s a problem with the DEF system on your diesel vehicle, we offer complete diagnostic and diesel repair services to get you back on the road quickly.