Diesel enthusiasts had a moment of doubt back in 2010 when the Environmental Protection Agency mandated the use of selective catalytic reduction (SCR) in diesel engines. They were required to use Diesel Engine Fluid (DEF) in order to regulate emissions coming from diesel engines. Being on uncharted territory, diesel enthusiasts had much to worry about, especially because of potential extra costs and maintenance requirements. Some of them even considered switching to gasoline engine vehicles, which speaks for itself.

Luckily, the thing with DEF sorted out rather quickly. It turned out that DEF is not that bad at all. Drivers soon realized that topping off an extra tank of liquid is worth the increased fuel efficiency and reduced emissions. But what is DEF in general and how does it work? We break down the topic for you in the sections below so you can better understand diesel exhaust fluid.

Diesel Exhaust Fluid

DEF stands for Diesel Exhaust Fluid, sometimes also called Aqueous Urea Solution and AdBlue. It’s a clear, colorless, and non-toxic liquid that adds to the exhaust stream of the engine to reduce emissions. In other words, DEF is an important component in keeping diesel vehicles and machinery within federal emission guidelines.

DEF features a combination of deionized water and urea, transforming nitrous oxide emissions into water and gas. Basically, the liquid works to make harmful emissions harmless in a couple of easy steps. This system is called Selective Catalytic Reduction (SCR), and it was issued the first time in 2010. Starting with this year, all major companies using heavy-duty diesel vehicles were installing SCR systems. It was all about meeting EPA emission standards that required reducing pollution output in diesel-burning engines.

Related Post: What is AdBlue and Why Do You Need It? 

Although the requirements first applied to trucks and other heavy-duty vehicles, soon the emission standard started to include off-road vehicles, generators, and different engines. DEF quickly became widely used and dominant on the market. Today, modern vehicles with SCR technology are adapted to provide easy DEF-level readings for the driver and alert on any sign of malfunction.

Exhaust pipe of car with low diesel exhaust fluid

  • General Information

At first, most drivers were concerned about DEF because of the possible extra expenses. They also thought that the liquid has to be added to the vehicle frequently, which is an additional drop in the sea of oil, additives, and other things already required for proper engine performance. However, it turned out that DEF is not that expensive at all, nor does it have to be used that often.

DEF should be refilled every time you change the oil. Therefore, it just adds to the regular maintenance you have to perform on your vehicle. Furthermore, DEF mostly contains natural components, so it’s not on the expensive side.

  • Where to Buy

You can get DEF at a lot of fuel stations as well as in online automotive stores. The product is not difficult to find at all, but you might want to double-check the certification. DEF features 32.5 percent of pure urea in purified water. If you want the SCR system in your vehicle to work properly, DEF has to meet an exact purity standard. Therefore, it’s crucial to get a product that comes from a certified manufacturer.

When buying DEF, look for the certification of the American Petroleum Institute (API), the German Institute of Standardization, and the International Organization for Standardization (ISO). The product should also meet AUS-32 specifications, so look for that label, too. If the product meets all these requirements, it features the proper concentration of urea and is safe for use.

  • How to Use

Refilling the DEF tank seems difficult, but the truth is, you can do it all on your own without much effort. Because DEF is widely distributed now, you can surely find it at your local auto parts store. After the purchase, you only need to find the fill port. It’s usually located next to the fuel filter under the hood or in the trunk. However, DEF is not to be mistaken for an oil or a fuel additive. The liquid never comes in contact with diesel. It is added to a different tank completely.

When it comes to the time of application, DEF should be refilled at the same time as the oil. This can vary a lot, depending on the vehicle operation and overall driving habits, but you can do a basic consumption count. For example, if your vehicle has a fuel efficiency of six miles per gallon and the dosing rate of three percent, it will use around one gallon of DEF per 200 miles.

Other Considerations

There are a couple of things you might want to consider before you start using DEF. Just like with any other fluid you add to your vehicle, DEF needs to be handled properly for the best efficiency and safety reasons. We made sure to include some of the most important considerations in the section above.

  • What if my vehicle runs out of DEF?

The biggest concern with the SCR system is the possibility of mixing the wrong fluids. If the system gets filled with incorrect fluid or contaminated in another way, it could collapse and cost you quite a bit. Therefore, it’s crucial to keep track of where each product goes, and always pour the liquids into the proper tanks.

Another thing that could cause you a lot of issues is a dry DEF tank. When your vehicle runs out of fluid, it will drop the speed scale to 5 miles per hour. You’ll be able to get to the nearest stop and refill the tank. However, keep in mind that once you turn off the vehicle, you won’t be able to start it again until the DEF tank is refilled.

Now, you won’t empty the DEF tank that easily. To prevent inconveniences on the road, manufacturers made sure to equip diesel vehicles with safety measures. You’ll have a gauge or a warning light to remind you of DEF levels. Both features will notify you if the level drops below 10 percent, so you’ll have enough time to fill the tank. Even if you forget about the warning the first time, you’ll have a couple of new ones coming for each mile passed. The warnings will get more frequent and brighter, and you definitely won’t miss them.

  • How to store DEF?

Once you pour DEF in your vehicle, you’ll want to store the rest of the product properly. That way, you’ll be able to use it again. DEF should be put in a cool and dry place, preferably in a well-ventilated area and far away from direct sunlight. The optimum storage temperature for DEF is up to 77 degrees Fahrenheit, although exposure to high temperatures doesn’t have a considerable impact on it.

Still, for longer shelf life, you might want to avoid exposing DEF to extremely high or low temperatures as well as direct sunlight. When the product is stored at between 10 and 90 degrees Fahrenheit, it will last around one year. If the temperature doesn’t go over 75 degrees Fahrenheit, DEF will last up to two years.

Another thing to consider is the fact that DEF contains pure components. Therefore, it’s essential to keep fluid uncontaminated. DEF shouldn’t be mixed with different fluids nor made in the garage by handypersons. Although DIYers would love to mix things up, DEF is the fluid sensitive to potential impurities. It’s difficult to get the exact combination of urea and water all on your own. Buying DEF from certified manufacturers is the smartest way to go.

Lastly, make sure to seal the cap of the container properly. DEF contains 67.5 percent of water, which means it will evaporate if exposed to high temperatures, and if left open. Otherwise, it’s not likely that DEF will evaporate over time.

  • Safety Measures

When it comes to the safety of your vehicle, it’s crucial not to mix diesel and DEF. We already mentioned that DEF shouldn’t be mixed with other fluids or contaminated in any way. It’s all the same with pouring DEF into the diesel tank. Both the SCR system and the engine in your vehicle will get ruined if you put these two fluids where they don’t belong. That will cost you thousands of dollars and, even worse, take you off your beloved vehicle.

Still, we all know that accidents happen. You can still make a mistake even with all the care you put into the process of separating the fluids. If you do mix the liquids, remember not to start the engine under any circumstance. You’ll need to completely drain the tank with the mixed fluids. When you get the job done, you can refill the tank with proper fluid and everything will be just fine.

Related Post: The Best DPF Cleaners 

  • Handling DEF

One of the main benefits of DEF is the fact it’s non-toxic, non-hazardous, and non-flammable fluid. Therefore, no special handling is required. The fluid is colorless, odorless, and safe to handle. It doesn’t pose a risk to humans and animals and is useful for equipment and the environment when used properly.

There are not many safety concerns when using DEF. If you spill the liquid on the floor or your vehicle, make sure to rinse it only with water. You might even inhale DEF while adding it to the tank. Although you should avoid it as much as you can, if you do inhale the fluid, step into some fresh air. That will help you avoid experiencing negative consequences.

However, people might react differently to various fluids. If any symptoms such as nose and throat irritation develop, make sure to see a doctor as soon as possible. Also, don’t ingest DEF. If that somehow occurs, make sure to get medical attention.

Technician replacing diesel exhaust fluid on car


The SCR system wasn’t only a burst that stayed in 2010. DEF is now widely used and it becomes more popular each year. Not only are big manufacturers such as Volkswagen and Ford implementing the SCR system, but a lot of other (and smaller) retailers are as well. DEF became a standard among diesel vehicle manufacturers, and it doesn’t seem that things will change in the future.

In fact, there’s no reason for changing good things. DEF is designed to help reduce levels of oxides of nitrogen emitted from diesel engines. It works to lower the production of emissions harmful to human health and the environment. Yes, it does make vehicle maintenance a little more difficult, but it’s well worth it.

When it comes to the benefits for the vehicle, DEF significantly improves fuel economy. With the help of SCR, the engine runs more efficiently and produces more power. Therefore, make no mistake that DEF is doing something bad to your diesel engine. The fluid actually improves the engine performance in a few levels, and you’ll notice it rather quickly.

In the end, you get to improve your vehicle’s performance, reduce fuel consumption, and take care of the environment all on your own. With DEF, you won’t be visiting the local mechanic that often.


  1. What is Diesel Exhaust Fluid? – CNET
  2. Diesel Exhaust Fluid – Wikipedia