What Does DEF Fluid Actually Do?

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a def fluid sign at a gas station

If you’ve recently purchased a diesel Ford F350 or a diesel Newmar Canyon Star, you have a lot of horsepower and torque to get you where you want to go. However, it’s not just diesel fuel that you’ll need when you fill up at the pump. You’ll also need to keep an eye on your DEF fluid. 

Do you know what DEF fluid is or what it actually does? Let’s learn more about this fluid’s necessity for your diesel engine’s function.

What Is DEF Fluid?

As a result of the Clean Air Act of 1970, the Environmental Protection Agency (EPA) was created to enact laws to address the environmental needs of America. In 2008, the EPA mandated that diesel engines require filters and biannual smog tests.

Manufacturers adapted to even more stringent requirements over the years, creating exhaust systems that use diesel exhaust fluid (DEF) to break down the generated harmful nitrogen oxides (NOx) into harmless nitrogen and water molecules. 

Thus, manufacturers could increase the horsepower and torque of diesel engines while cutting down on NOx emissions.

Chemically, DEF fluid is 67.5% deionized water and 32.5% urea. In basic terms, when DEF is injected into the hot exhaust gas, the water evaporates, leaving the ammonia and isocyanic acid from the urea compound. 

Ammonia reduces the harmful nitrogen oxides. In essence, it cleans the exhaust gas before releasing it into the air.

What Vehicles Need DEF Fluid?

All diesel-powered vehicles manufactured after Jan. 1, 2010, require DEF fluid to comply with EPA standards.

This includes trucks and large pickups, delivery trucks, buses, RVs, semis, and light-duty vehicles such as minivans and SUVs. Older vehicles produced before the EPA requirements are grandfathered in.

What Does DEF Fluid Do?

In more complicated terms, DEF fluid comprises (NH2)2CO. Once this compound heats, it breaks down into ammonia and isocyanic acid. Then the acid breaks down in water to become carbon dioxide and ammonia. This is where ammonia begins breaking down the nitrogen oxides.

Once the urea has worked its magic through this chemical process, what’s released through the exhaust is much safer for the environment and meets emissions requirements set by the EPA. By using DEF fluid, manufacturers can create as powerful an engine as they want while still abiding by EPA guidelines.

How Often Do I Need to Fill Up the DEF Tank?

How often you fill your DEF tank depends on how often you drive your diesel vehicle and for what purposes. If you tow heavy loads, you’ll fill up at the fuel pump and must top off the DEF fluid more frequently.

Each truck is also different. For example, a newer truck will be more efficient than an older one. A light-duty truck will use less than a heavy-duty truck. 

Generally, you can expect a light-duty truck to consume two or three gallons of DEF per 800 to 1,000 miles. Larger vehicles will use an average of a gallon of DEF for every 50 gallons of diesel fuel burned.

Keep in Mind: What happens when you put Diesel in a gas engine? If you end up making this mistake, make sure you know what to do!

What Happens If I Run Out of DEF Fluid?

When it’s time to fill the DEF tank, you’ll receive an alert on the dashboard of your vehicle. Paying attention to this signal will give you plenty of time to fill up. If you don’t, you risk running out of DEF fluid. 

Beware of running out. Your vehicle will start running on a lower power mode because it’s trying to conserve DEF. Once you shut off your engine, it won’t restart until you refill the DEF tank. However, this shouldn’t happen if you heed the alerts and warnings from your vehicle’s system.

close up of a diesel cap for def fluid

Where Can I Buy DEF Fluid?

Most truck stops that have diesel fuel will also have DEF fluid. You’ll find the DEF pump connected to the same fuel pump as the diesel pumps. A flap normally covers it up. The handle is generally blue to help distinguish it from the green diesel handle. 

DEF usually costs about $2 to $3 per gallon at the pump. Some convenience stores will sell DEF in large jugs inside. Other locations that sell large containers include Walmart, Target, NAPA, Advance Auto Parts, and O’Reilly’s. 

Generally, you’ll find that DEF will cost $11 to $15 for about 2.5 gallons. So it’s better to fill up at the pump to save money.

Keep in Mind: Before you purchase DEF fluid form Walmart, read these 5 Reasons we recommend avoiding Walmart DEF Fluid

BlueDEF Diesel Exhaust Fluid Synthetic Urea & Deionized Water 2.5 Gallon Jug
  • ENGINEERED: For use in Selective Catalytic Reduction (SCR) systems on diesel engines
  • EFFICIENCY: BlueDEF diesel exhaust fluid is key to helping improve fuel economy by up to five percent

Will DEF Fluid Freeze?

DEF fluid can freeze. But the good news is that when temperatures rise, and it thaws, it regains its effectiveness. The freezing and thawing of the DEF fluid don’t diminish its quality.

But keep an eye on the temperatures in the winter. When they drop below 12 degrees Fahrenheit, your DEF fluid will begin to solidify and crystallize.

Diesel engines may not start in very cold temperatures. However, many have components that help heat the DEF tank. For example, the SCR (Selective Catalytic Reduction) systems heat the DEF tank so it thaws and can start working efficiently.

Take Care of Your Diesel Engine and Keep an Eye on Your DEF Fluid

When you purchase a diesel-powered vehicle, you might seek more horsepower or better fuel economy. But don’t forget proper maintenance to ensure its longevity. 

You’ve spent a lot of money upfront on a diesel engine. Pay attention to your DEF level just like you do your fuel. You don’t want to end up stranded on the side of the road, especially if you have a trailer in tow.

Do you know the level of your DEF tank?

Last update on 2024-07-21 / Affiliate links / Images from Amazon Product Advertising API

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