What Does DEF Stand For? What Does It Do?

Diesel cars are powerful and can handle demanding tasks over long traveling distances, but their increasing emissions are quite concerning. 

Therefore, professionals have introduced several additives and products to help address the issue, including the DEF liquid. Scroll to learn what it is and when it is required.

What Is DEF?

DEF stands for diesel exhaust fluid, which you can find in many diesel-powered vehicles. It is an odorless, non-toxic solution made of pure automotive-grade urea and purified water, integrated into the engine’s exhaust system to reduce NOx (nitrous oxide) emissions. 

Specifically, DEF combines with the SCR (selective catalytic reduction) to break the NOx molecules into clean water vapor and nitrogen. As a result, diesel vehicles can comply with environmental regulations set by local authorities or the EPA (Environmental Protection Agency).

diesel exhaust fluid meaning

Three important things you should keep in mind:

  • DEF is not a fuel additive; it’s only designed for emissions reduction.
  • It is stored in a separate tank in the vehicle, not mixed with diesel.
  • Although the consumption rate of DEF depends on your vehicle model and operating conditions, the number typically ranges from 2-3% of diesel’s consumption. 

How Does Diesel Exhaust Fluid Work?

As mentioned, DEF works closely with SCR, a new technology in diesel engines that uses catalytic converters and urea-based solutions to reduce nitrogen oxides. SCR functions as an after-treatment system, meaning the exhaust gasses will be handled after exiting the engine and before being released into the air. 

Below is a summary of the process:

Stage 1. DEF is injected into a tank separated from the diesel in the vehicle. 

Stage 2. A small amount of DEF enters the exhaust stream as your exhaust gasses travel through the SCR system. Inside the SCR catalyst chamber, high temperatures from the gas cause the DEF to vaporize rapidly.

Stage 3. The vaporized DEF decomposes into ammonia (NH3) and carbon dioxide (CO2). The ammonium content acts as a catalyst during NOx’s conversion into harmless nitrogen (N2) and water vapor (H2O), hence the absence of harmful pollutants in the exhaust. 

Diesel Exhaust Fluid Is Required for All Diesel Models

Due to evolving emission standards, DEF has become compulsory for all diesel vehicles manufactured since 2010 for private or commercial use, regardless of small or large, to comply with the EPA regulations

Plus, according to The Clean Air Act, offering, manufacturing, installing, or selling components or parts that bypass DEF and other emissions controls is illegal. 

In recent years, modern vehicles have also taken further steps to ensure their DEF level is always sufficient. One example is the monitoring system for keeping track of the DEF volume; an empty DEF tank or a low DEF level will shut the car down or limit your driving speed. In some cases, your engine will not start until the DEF has been refilled.

Does It Go Bad? How Long Does It Last

Yes, DEF does go bad. It usually expires after 1 year, assuming the liquid has not been severely contaminated before that. 

Fortunately, DEF is colorless and clear, so identifying symptoms of contaminated or expired fluid is easy, even for beginners. Simply inspect the DEF closely to spot foreign particles, usually from dirty container nozzles or funnels you used to pour other liquids. Its color might also turn cloudy or darker. 

To prevent the DEF from going bad long before the expiration date, remember that proper storage is the key:

  • Use containers made of stainless steel or polyethylene approved by the ISO. Avoid using fuel containers made of/containing aluminum since DEF might corrode them.
  • Direct sunlight or high heat degrades DEF quite fast. By contrast, frozen, slushy DEF previously stored in low temperatures can still be used, so always choose a dry and cool place for your storage. 
  • If you live in areas with dramatically fluctuating temperatures, always track the changes through monitors, gauges, etc., to detect issues with the DEF as early as possible. 
  • Do not fill your containers completely; leave some space for the DEF to expand. 

Change DEF Often To Ensure Efficiency

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As mentioned, DEF consumption usually counts for 2-3% of the diesel consumed. We recommend refilling the DEF every 3 or 4 times you fill up your larger diesel tank. Still, you can always choose much shorter intervals if desired. 

Remember to stick to your schedule, as neglecting or forgetting to change the DEF when it is due will lead to numerous issues:

  • Bad engine performance. The engine derates due to excessive harmful emissions, affecting fuel efficiency and acceleration. 
  • Legal compliance. DEF is compulsory for passenger cars and commercial vehicles, so operating without it will definitely result in potential penalties and fines.
  • Environmental impacts. Without DEF, the SCR cannot turn NOx into water vapor and nitrogen. The toxic NOx emissions will pollute the air, leading to more environmental issues. 
  • Costly repairs. The lack of DEF will damage your SCR system components, such as the catalysts, pumps, and DEF injectors. Fixing them all can cost thousands of dollars.


This guide has explained what diesel exhaust fluid does and the problems you might encounter if driving with little or no DEF liquid in the SCR system. 

So, monitor and change the DEF frequently. Do not forget our extra storage tips, either, which help preserve the liquid’s quality far better than imagined.

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Alex Lewis

Alex Lewis

Petroleum Engineer At Rex Energy

I have worked in a variety of roles and professions, from quality engineering in the automotive industry to production engineer in the oil and gas sector. From a technical point of view, these roles have shown me how to design a process, ensure it is efficient and up to standard, and manage the execution of the said process from start to finish.

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