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Best Diesel Exhaust Fluids

Updated November 2018
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BestReviews spends thousands of hours researching, analyzing, and testing products to recommend the best picks for most consumers. We buy all products with our own funds, and we never accept free products from manufacturers.
BestReviews spends thousands of hours researching, analyzing, and testing products to recommend the best picks for most consumers.
BestReviews spends thousands of hours researching, analyzing, and testing products to recommend the best picks for most consumers. We buy all products with our own funds, and we never accept free products from manufacturers.
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We purchase every product we review with our own funds — we never accept anything from product manufacturers.

  • 14 Models Considered
  • 6 Hours Researched
  • 1 Experts Interviewed
  • 90 Consumers Consulted
  • Zero products received from manufacturers.

    We purchase every product we review with our own funds — we never accept anything from product manufacturers.

    Why trust BestReviews?
    BestReviews spends thousands of hours researching, analyzing, and testing products to recommend the best picks for most consumers. We buy all products with our own funds, and we never accept free products from manufacturers.
    BestReviews spends thousands of hours researching, analyzing, and testing products to recommend the best picks for most consumers.
    BestReviews spends thousands of hours researching, analyzing, and testing products to recommend the best picks for most consumers. We buy all products with our own funds, and we never accept free products from manufacturers.

    Shopping guide for best diesel exhaust fluids

    Last Updated November 2018

    If you’re new to vehicles that run on diesel fuel, you’ll soon discover they’re quite a bit different from standard gasoline-powered vehicles, beyond the difference in fuels. For example, you might need to change the oil and fuel filters more frequently. Because a diesel engine burns fuel at a higher temperature, you might have to use an exterior heating system in the wintertime. But the most obvious difference is that you need to add diesel exhaust fluid to a separate tank on the diesel vehicle to remove harmful emissions and enable it to run properly.

    Purchasing the best digital exhaust fluid for your situation depends on a few criteria. At BestReviews, we can help you better understand the differences among diesel exhaust fluids, so you can find the best one for your vehicle.

    Our shopping guide gives you plenty of helpful information about diesel exhaust fluids, so you can make sure your diesel vehicle runs properly.

    Because the diesel exhaust fluid is added to a truck’s exhaust system, it doesn’t affect the vehicle’s gas mileage.

    What is diesel exhaust fluid?

    Unless you have experience with diesel vehicles, you might not know much about diesel exhaust fluid (DEF). This bright blue liquid is a mixture of roughly two-thirds deionized water and one-third urea. The color helps you distinguish it from gasoline or diesel fuel.

    You add DEF to your vehicle as needed. It goes into a separate tank than the diesel fuel and runs through the selective catalytic reduction (SCR) system. Diesel vehicles emit dangerous nitrogen oxide (NOx). DEF breaks down the NOx emissions into nitrogen and oxygen, elements commonly found in air.

    The DEF you use must meet ISO 22241, which is an international standard that determines the quality of the product.

    Who needs DEF?

    If you have a 2010 or newer model vehicle that runs on diesel fuel, it almost certainly needs DEF. A vehicle that needs DEF is designed so that it won’t operate properly without DEF in the tank.

    If you’re unsure whether your vehicle requires diesel exhaust fluid, look for a special DEF tank. This tank is near but separate from the diesel fuel tank. DEF and diesel fuel should never mix.

    What happens if you run out of DEF?

    Because diesel exhaust fluid is an expensive add-on, and because it can be a hassle to keep on hand, it might be tempting to run the diesel vehicle without filling the DEF tank. However, the vehicle manufacturer is required to design the vehicle so you can’t.

    The Environmental Protection Agency (EPA) sets the rules that vehicle manufacturers must follow regarding the use of DEF. Under these rules, dashboard messages and warning lights warn the driver when the DEF tank is running low on fluid.

    An empty DEF tank limits the vehicle's performance to speeds of up to five miles per hour until the tank is refilled.

    What happens if you put fuel or DEF in the wrong tank?

    Let’s just say you don’t want to put the wrong fluid in the wrong tank! It will result in a significant repair expense for you. Be very careful when adding either diesel fuel or DEF to your truck.

    • Diesel fuel in DEF tank: There are safeguards built into the system to prevent you from putting diesel fuel into the DEF tank. The diesel fuel nozzle is larger in diameter than the entry hole of the DEF tank, ensuring that you can’t accidentally fill the DEF tank with diesel fuel using the nozzle at a gas station. Also, the DEF tank often has a bright blue cap that will help you avoid placing diesel fuel in this tank.
      However, if you’re inadvertently put diesel fuel in the DEF tank, you need to have the system repaired immediately. Even a tiny amount of diesel fuel could significantly damage the system that uses DEF.

    • DEF in diesel fuel tank: You don’t have any built-in safeguards against accidentally adding DEF to the diesel fuel tank other than using common sense. If you inadvertently place diesel exhaust fluid into your diesel fuel tank, your vehicle won’t operate. The entire fuel system will need to be professionally cleaned and repaired.

    How DEF brands differ

    It can be frustrating to pick the right DEF for your vehicle. All diesel exhaust fluid must have a urea concentration of 32.5%, so how different can the brands be?

    • Impurities: To meet EPA standards, DEF must contain a limited percentage of impurities such as iron, zinc, and aluminum. These impurities can enter the DEF if the manufacturer uses poor-quality deionized water, for example. Too many impurities in the DEF could cause the SCR system to fail prematurely. A cheaper brand of DEF might be at the edges of the impurity limits, while a more expensive DEF might have many fewer impurities than required by the EPA.

    • Urea: An approved brand of DEF must use pharmaceutical-grade urea rather than agricultural-grade urea, which might have impurities like biuret in higher levels than higher-quality urea.

    • EPA standards: To call itself diesel exhaust fluid, the product must meet all EPA standards, especially ISO 22241. If you find a cheap product that claims to be diesel exhaust fluid, but it doesn’t meet ISO 22241 standards, don’t purchase it. Such a fluid could damage your vehicle’s SCR system.

    • Manufacturer recommended: When looking for a brand of DEF to purchase, check your vehicle owner’s manual. It might recommend a particular brand, and you should follow this recommendation. If you don’t have a manufacturer recommendation, only select a product that meets ISO 22241 standards to protect your SCR system.

    You can switch brands of DEF at any time. Mixing different brands of DEF in the DEF tank will not harm the SCR system as long as both brands follow the standards, particularly ISO 22241.

    DID YOU KNOW?

    The amount of diesel fuel you use will determine the amount of DEF you use. The SCR system in your vehicle uses a ratio of between 50 to 1 and 35 to 1 of diesel fuel to DEF.

    DID YOU KNOW?

    Because of the concentration of urea to deionized water in DEF, it has a low freezing point (12°F).

    Diesel exhaust fluid prices

    The prices for DEF aren’t set in stone and fluctuate over time depending on market conditions, supply and demand, your location, and the size of the container. You can expect to pay between $2.50 and $8 per gallon for DEF.

    One- to five-gallon containers: You can expect to pay between $4 and $8 per gallon for containers of this size.

    55-gallon drums: When you buy more, you can expect to pay a little less, between $3 and $5.50 per gallon.

    275- or 330-gallon containers: Commercial operations will pay prices similar to that of diesel fuel, between $2.50 and $4 per gallon, for large quantities of DEF.

    DEF containers have an expiration date printed on them. Most DEF fluids have a shelf life of about two years.

    FAQ

    Q. Is DEF a diesel fuel additive?

    A. Technically, diesel exhaust fluid is not an additive because it isn’t mixed with the diesel fuel. Instead, the DEF is poured into a separate tank on your diesel fuel vehicle. The SCR system built into the vehicle then applies the DEF to the exhaust system as needed to eliminate NOx emissions.

    Q. Why have I never heard of DEF or SCR? I’ve driven pickup trucks most of my life.

    A. The use of diesel exhaust fluid in a selective catalytic reduction system has only occurred in the last several years. If you’ve always driven older trucks, you haven’t had to use DEF because your trucks don’t have an SCR system. This system began appearing on diesel vehicles with the 2010 model year.

    Q. How do I know if my vehicle needs diesel exhaust fluid?

    A. If your vehicle uses diesel fuel and has an SCR system, you need DEF. Among vehicles commonly aimed at consumers, pickup trucks and some sport utility vehicles (SUV) run on diesel fuel, although large sedans occasionally use diesel, too. A vehicle with an SCR system has a special tank for the diesel exhaust fluid. This DEF tank, separate from the diesel fuel tank, is usually distinguished by a bright blue cap.

    Q. How does the selective catalytic reduction (SCR) system work?

    A. The system uses chemical reactions between the diesel exhaust fluid and the vehicle’s exhaust to reduce harmful emissions. The SCR system uses an injection system to deliver the diesel exhaust fluid into the vehicle’s exhaust system. The DEF is then converted into ammonia by the SCR system. Once the ammonia is mixed with the NOx emissions, it breaks them down into safe emissions of nitrogen and oxygen.

    The team that worked on this review
    • Bronwyn
      Bronwyn
      Editor
    • Devangana
      Devangana
      Web Producer
    • Eliza
      Eliza
      Production Manager
    • Kyle
      Kyle
      Writer
    • Linsay
      Linsay
      Editor
    • Melinda
      Melinda
      Web Producer

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