What Is Diesel Exhaust Fluid (DEF)?

In 2007, the EPA (Environmental Protection Agency) starting putting out legislation to hugely reduce diesel emissions from heavy-duty trucks 1.

This lead to the introduction of the SCR (Selective Catalyst Reduction) and DEF (Diesel Exhaust Fluid) in most heavy duty diesel pick-ups by 2013. Some manufacturers, such as General Motors, introduced the system a little earlier, with the Duramax getting it in 2011.

What is DEF?

DEF is also known as UREA because it is a solution made of de-ionized water and urea. The DEF fluid is sprayed into the exhaust fumes to break down emissions. NOx (Nitrogen Oxide) is poisonous chemical produced as a result of combustion in diesel engines and causes pollution. The DEF causes a chemical reaction that turns the NOx into harmless nitrogen and water 2.

System Overview

The DEF injection system has the following parts:

• DEF Tank
• DEF Pump
• DEF injector
• heated DEF lines
• NOx sensors
• Temperature sensors
• SCR catalyst

When do you need DEF?

DEF fluid is essential to the system just as diesel is, your truck will not run right without it and should not be operated when DEF is empty as this can cause major problems. If DEF levels are extremely low or empty, the vehicle may go into limp mode, with speed restrictions and not start again when shut off.  Every DEF equipped vehicle will have a level gauge of sorts or an alert to let you know when you get low. The fluid is available from most vehicle dealers, gas stations or major supermarkets.

How do you fill up?

There is a filler (separate from your diesel filler). In Ram vehicles, it is located behind the fuel door on the driver side of the vehicle with a blue cap. Duramax (GMC & Chevy) trucks have the DEF filler under the hood on the passenger side, also with a blue cap. Cab and Chassis Trucks (ie. flatbed) have the filler between the cab and the chassis. When running low or empty always add at least 2 gallons. If you spill DEF on the engine compartment, use a damp cloth with water to wipe away.

How long does a tank last?

All factors being good, a tank of diesel exhaust fluid should last for around a few thousand miles. Several factors may affect this though. If the engine is running under load, for example when towing, then more DEF fluid will be used as the engine consumes more fuel. Another factor that will greatly affect how much is used, is the temperature.

Does temperature affect DEF?

Yes. Because the solution is 67.5% Water (32.5 urea), when stored, it is vulnerable to evaporation. Storing containers at 95°F for an extended period reduces the shelf life to 6 months. ” If stored between 10 and 90 deg F, shelf life will easily be one year. If the maximum temperature does not exceed approximately 75 deg F for an extended period of time, the shelf life will be two years” 3.

The solution will freeze at around 12°F. Both the urea and water will freeze and thaw at the same rate, but will not cause degeneration of the solution unlike exposure to extremely hot temperatures will. This is why the system lines are heated, to ensure the solution is the optimum temperature in cold climates. Do not add anti-gelling or freeze point improver as this may cause damage to the entire SCR system.

Can you mix brands of DEF?

Yes, you can mix brands as they all have to meet the same standardized criteria. The packaging should display the API (American Petroleum Institute certification. Also the “German Institute of Standardization DIN70700, the International Organization for Standardization ISO 22241-1, and meet AUS – 32 specifications. This will ensure the proper purity and concentration (32.5%) of urea” 3. As previously mentioned, though, the temperature at which its stored will affect the

Do you have any other DEF questions? Let us know, comment below!