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What Is Diesel Exhaust Fluid And How Does It Work?

Did you know that when diesel fuel burns, it releases polluting nitrogen oxide (NOx) emissions into the air? Those emissions can cause serious health issues, like asthma attacks or lung cancer.

That's why in 2010, the U.S. Environmental Protection Agency tightened up its emissions standards for diesel vehicles. Diesel vehicles were no longer allowed to release as much NOx emissions into the air.

Automakers needed to meet those rigorous emissions standards. They started using diesel exhaust fluid (DEF) in their diesel vehicles. DEF considerably reduces these emissions, by up to 85%. In other words, DEF helps vehicles meet the emissions standards. That's why DEF is used in many diesel-powered vehicles across the world.

As well as being eco friendly, DEF also helps vehicle owners save money on fuel costs due its ability to produce less emissions. In fact, studies show that using DEF can help diesel drivers save up to 5% of their total fuel costs!

Diesel Exhaust Fluid: What It Is And How It Works

Adding diesel exhaust fluid

DEF is a non-hazardous fluid. It contains:

  • 32.5% urea
  • 67.5% de-ionized water

DEF is used in diesel engines (such as the Duramax engine) found in passenger vehicles. Contrary to popular belief, DEF is not a fuel system additive. It never comes into contact with diesel. Rather, it's stored in a separate tank and injected into the exhaust stream.

How DEF Works

DEF is an important part of Selective Catalytic Reduction (SCR) technology. This technology was patented in 1957, but it only started becoming widespread after 2010. You can find this technology in pretty much all diesel trucks and SUVs produced after 2010.

SCR technology exists to reduce NOx emissions. It does this by breaking down NOx emissions into a harmless mixture of nitrogen and water. For NOx to turn into nitrogen and water, it needs ammonia. That's where DEF comes in. DEF contains ammonia (NH3).

There are sensors in the exhaust system that monitor the amount of NOx emitted by the engine. If the NOx levels become too high, DEF injects into the exhaust stream.

When DEF is injected, the ammonia (NH3) in the diesel exhaust fluid reacts with the nitrogen oxides (NOx) in the exhaust. This reaction breaks down both components into nitrogen and water vapor. These are harmless.

Is Diesel Exhaust Fluid Dangerous?

DEF fluid cap

Not at all. DEF is the least hazardous fluid out of all the fluids used in a truck, such as:

  • Motor oil
  • Brake fluid
  • Windshield wiper fluid
  • Diesel fuel
  • Antifreeze

DEF is corrosive, though, for some metals. So you should be careful not to store DEF in containers made of:

  • Carbon steel
  • Copper
  • Aluminum
  • Zinc

What Happens When Your Truck Runs Out Of Diesel Exhaust Fluid?

When your truck runs out of DEF, the SCR system will enter "degraded mode". In this mode, emissions levels will be high and the vehicle speed is limited to a very low speed, sometimes 5 mph. The driver should then stop driving their vehicle and refill the tank with DEF as soon as possible. This happens because the U.S. Environmental Protection Agency states that vehicles can't run without DEF.

If you own a diesel truck, you won't be blindsided when it runs out of DEF. You'll receive plenty of warnings beforehand. If the DEF tank drops below a certain percentage, such as 10%, you'll get a warning on your dashboard. In some GM diesel trucks, you can find out how much DEF is left (in terms of mileage or percentage of the fluid left in the tank).

If you get such a warning, replenish your truck's DEF supply as soon as possible. You can buy DEF from many online and in-store retailers such as AutoZone or Walmart. You can also find it at a gas station.