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What’s the Point of DEF Fluid in Your Truck?

What’s the Point of DEF Fluid in Your Truck?

If you own a diesel truck, there’s a good chance that it requires diesel exhaust fluid (DEF). The Environmental Protection Agency (EPA) began introducing legislation and mandates in 2008 that tightened the requirements and restrictions for vehicle manufacturers.

DEF became common on trucks with diesel engines at this time.

You might be wondering what the point of DEF fluid is. Let’s take a look!

What Is DEF?

DEF is a liquid mixture of urea (32.5%) and water (67.5%) injected into a diesel vehicle’s exhaust stream. The fluid plays an integral role in the Selective Catalytic Reduction (SCR) system. If you’re looking to purchase a diesel truck, it will likely come with this system.

Some truck owners pay professionals thousands of dollars to remove the SCR system and the need for DEF. However, this can cause a few issues. You may not be able to pass an emissions test. Not to mention, there may be legal ramifications.

Removing the SCR system can also void your vehicle’s warranty and put you on the wrong side of the law. Many dealers won’t accept a car with the DEF deleted. It’s better to be safe than sorry and leave the SCR alone.

What’s the Point of DEF Fluid?

Despite being 67.5% water, DEF fluid helps reduce the number of nitrogen oxides from medium and heavy-duty vehicles with diesel engines. The fluid helps diesel vehicles meet the emissions requirements set by the EPA but doesn’t compromise the performance or fuel efficiency of the car.

Pro Tip: Do you have a diesel truck? If you’ve found water in your fuel, this is How to Get Water Out of Diesel Fuel.

What Is Selective Catalytic Reduction (SCR)?

The Selective Catalytic Reduction (SCR) system converts nitrogen oxides into nitrogen and carbon dioxide. While SCR systems are relatively new on passenger vehicles, the technology has been around since the 1950s. S

CRs on cars can help protect the environment by reducing harmful emissions.

Do All Diesels Use DEF Fluid?

The EPA requires that all diesel engines manufactured after January 1, 2010, meet their new requirement. All heavy-duty engine manufacturers, except Caterpillar and Navistar International, use SCR technologies that require DEF fluid. You’ll find DEF in Ford, RAM, and Chevrolet trucks.

You’ll also find conversion vans and passenger diesel cars that use SCR technology.

How Often Do You Need to Add DEF Fluid?

How often you’ll need to top off your DEF tank will depend on your engine and the load you’re hauling. You may fill up on DEF fluid more if you’re towing heavy cargo.

However, if you’re not regularly towing or hauling heavy loads, you’ll likely get 6,000 to 8,000 miles on a tank. You’ll probably expect 3,000 to 4,500 miles while you are towing.

Because there are so many variables regarding your truck’s size, the trailer you’re hauling, and if you’re driving through mountains or the plains, it can be somewhat unpredictable.

Diesel exhaust system
DEF is injected into a diesel vehicle’s exhaust stream.

How Many Miles Does a Gallon of DEF Last?

There is no magic formula for miles per gallon of DEF fluid. However, the more fuel you use, the more DEF you will use.

Many trucks can get 18-22 mpg of fuel when not towing but get 8-12 mpg when towing. So driving your vehicle hard will cause you to use more DEF. 

What Happens If You Run Out of DEF Fluid?

You’ll get a notification from your vehicle when you have approximately 500 miles until you’re out of DEF fluid. That’s typically plenty of time to find a spot to get some DEF in your tank.

However, if you ignore the warning and any subsequent warnings, your vehicle speed will start to run on a lower power mode. This will reduce the power to help conserve the remaining DEF in your tank. 

The vehicle will remain in this low power mode and stay running until you shut off the engine. If you don’t put DEF in the tank, the engine will not start. You’ll get plenty of warnings and dashboard indications that will give you plenty of time to find DEF fluid. So make sure you don’t put it off, or you’ll risk getting stranded.

Pro Tip: Want to jump from a diesel truck to an RV? Find out more about What Is A Diesel Pusher RV?

Person filling up their truck with diesel.
If you use a diesel truck, you’ll need to know about DEF fluid.

Where Can You Buy DEF Fluid?

You can buy DEF at many gas stations, truck stops, and even big-box retailers. You’ll find DEF in a jug at many of these locations. However, the more cost-effective solution is to buy your DEF at the pump. This is usually how semi-trucks and other vehicles that use large quantities of DEF will fill their tanks.

How Much Does DEF Cost?

Much like gasoline, DEF can vary in price depending on where you’re trying to fill your tank. From many big-box retailers like Walmart, you can typically expect to pay $11 to $15 for approximately 2.5 gallons. This commonly equates to around $6.00 per gallon of DEF.

However, getting your DEF at the pump is a much more cost-effective option. You can generally purchase it for $2 to $3 per gallon.

Not only does this give you the best bang for your buck, but it’s also more convenient. Just like when you’re filling up with fuel, the DEF pump will shut off when your tank is full. This means you’ll know that your tank is full, and you’ll be good to go for several thousand more miles.

Don’t Forget to Refill Your DEF Tank!

You don’t want to forget to keep DEF fluid in your tank. If you don’t, you’ll be left stranded on the side of the road or in a parking lot looking for DEF.

We strongly suggest filling up as you’re stopping for fuel at a truck stop. Fill your tank and make sure you are out of the way of any semi-trucks. It’s as easy as that, and you’ll be in and out with a full DEF tank in no time.

Have you ever had a close call when it comes to DEF? Drop a comment below!

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Bob Ewing

Sunday 20th of February 2022

I think it's terable I have 6 truck that don't require it that' are 2008 and earlier they get 25 mpg plus the one truck 2020 that requires it only gets 12 mpg. What's wrong with this has more wasted fuel and has to polute more if it wasted 10 mpg. For nothing except fuel companies make more money


Friday 18th of February 2022

DEF does infact impact your fuel economy as its necessary for the vehicle to operate and its consumable.. Therefore it needs to be calculated into your fuel purchase. Especially if you are a Semi owner operator.


Friday 18th of February 2022

they claim that def is killing the honey bee, if this is true we will perish also. trucks ship bees.

Southern feller

Friday 18th of February 2022

I get 28 mpg all day long eith a tuned and deleted 6.7 Cummins. The emissions crap they put on trucks cause extra wear and tear. Lower fuel mileage and extra cost when it comes to maintenance and repairs. Many states don't even require emmisions test anymore and I've never had a single officer of any type bring it up.

Just pay to have it deleted out, it's well worth it and it's fun to flip off all those tree hugging hippie wannabes at the epa.

Dean H

Thursday 17th of February 2022

I absolutely agree with other comments. Emissions systems requiring def are a maintenance nightmare on cost and downtime. Mileage reduction is at least 30% and more towing. The net gain on pollution is actually a net loss when figuring extra fuel used and a greater reliance on foreign fuel. There really are no positive aspects except feel good factor to uninformed and those that sell the urine for conversion

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