Blog Car Care Advice What is Diesel Exhaust Fluid and Why Is It Important?
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What is Diesel Exhaust Fluid and Why Is It Important?

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The Environmental Protection Agency (EPA) mandated that all diesel engines must meet certain emission requirements by using Selective Catalytic Reduction (SCR). That’s why diesel engine manufacturers started using diesel exhaust fluid to reduce NOx (Nitrogen Oxide) emissions. 

Wondering what diesel exhaust fluid is?
And is it necessary to use DEF for your diesel engine?

Fret not! 
We’ve got all the details about diesel exhaust fluid, including what your DEF light means, and the answers to some frequently asked questions

This Article Contains

Let’s get started.

What Is Diesel Exhaust Fluid?

Diesel exhaust fluid (DEF) is a clear liquid that reduces the harmful emissions a diesel engine produces. Diesel exhaust fluid is a urea solution regulated by the American Petroleum Institute (API), comprising 67.5% deionized water and 32.5% urea. 

Here’s a breakdown:
Urea is a refined compound also found in fertilizers and deionized water is, well, water without ions. 

Note: Vehicles undergoing SCR have a separate fuel tank and DEF tank. Your DEF tank is insulated, heated, and marked by a blue filler cap.

Now that we know what DEF fluid is, let’s see why you need it.

Why Is Diesel Exhaust Fluid Necessary?

Today, most modern diesel engines offer increased fuel efficiency and reduced emissions. 

They combine processes like selective catalytic reduction and exhaust gas recirculation. They also use a diesel particulate filter that catches particulate matter

Here’s how SCR breaks down the nitrogen oxide (NOx emissions) into harmless nitrogen and water vapor:

  1. First, the engine’s exhaust gas moves through a particulate filter, catching all the particulate matter like soot and ash from burning impure fuel. 
  2. The exhaust gas moves past a nozzle that sprays diesel exhaust fluid into the exhaust stream of gasses.
  3. Next, the hot exhaust gas-DEF mixture enters your catalytic converter. 
  4. The urea and the harmful gas react with various metallic catalysts inside your catalytic converter
  5. The heat in the catalytic converter turns DEF’s urea into isocyanic acid and ammonia.
  6. The resulting isocyanic acid mixes with water (from the DEF), producing more ammonia.
  7. The ammonia and oxygen then react with nitrogen oxide to produce harmless nitrogen gas, water vapor, and carbon dioxide. 
  8. These engine emissions then leave your vehicle’s exhaust and enter the atmosphere. 

This process is similar to how your gasoline-powered car’s catalytic converter works. The only difference is that diesel engines inject urea into the exhaust stream.

Keep scrolling to learn more about DEF! 

4 Essential Things to Note About Diesel Exhaust Fluid

Here are some lesser-known aspects of diesel exhaust fluid:

1. Only Use Recommended Materials When Handling DEF

Think of DEF fluid as highly allergic to everything other than titanium, rubber, plastic, and stainless steel. If it comes into contact with incompatible compounds, there’s a high risk of contamination. 

For DEF packaging and equipment, you must strictly avoid brass, copper, epoxy resins, and nickel.

2. Freeze/Thaw Cycles Won’t Affect DEF

Although DEF shelf life is affected by temperature, cycles of freezing or thawing won’t impact your DEF system. The DEF tank is wrapped in heating coils to let the product thaw rapidly once you start your engine

3. Color Change Indicates Contaminated DEF

Anything other than a clear liquid means that you have a contaminated fluid. For example, when DEF comes into contact with brass or copper, it’ll appear light blue; when in contact with steel or galvanized steel, it will turn a rusty color.

4. You Can’t Restore Contaminated DEF

Once DEF gets contaminated, you can no longer use it. You must replace it to ensure your diesel pickup truck meets the emission requirements and doesn’t experience further damage. 

Since you know the ins and outs of diesel exhaust fluid, let’s learn more about your DEF dashboard light. 

What Your DEF Warning Light Means

By now, you should have a good understanding of your DEF system. Any abnormality in the DEF system triggers your DEF light or ‘Adblue’ light. A DEF system malfunction can affect your engine’s performance, so you must diagnose the issue ASAP. 

Here are some common issues that trigger your DEF light:

Your best bet at resolving the DEF dashboard light issue is to call a mechanic. They can run diagnostics, make the proper repairs, and have you on the road promptly. 

Still have some unanswered questions about diesel exhaust fluid? 
Let’s dive right in.

5 Diesel Exhaust Fluid FAQs

Here are the answers to five frequently asked questions regarding exhaust fluid.

1. How Often Should I Fill My DEF Tank?

For standard vehicles, you should fill your DEF fuel tank once every 3 to 4 times that you refuel with diesel fuel. The frequency can vary with operating conditions and your fuel consumption.

For a light-duty diesel truck, it depends on miles per gallon and its usage. According to some OEMs (Original Equipment Manufacturers), a light-duty truck will use around 2-3 gallons of DEF every 800 miles.

However, trucks with an average miles per gallon rating of 20+ can go around 8,000 to 10,000 miles on a tank full (10 gallons) of DEF.

2. Can My Diesel Engine Run Without DEF? 

Well, yes and no. Here’s why:

The Environmental Protection Agency (EPA) ensures that all diesel engine manufacturers integrate a warning system to notify drivers of the status of their DEF system. 

If you ignore this warning, your diesel truck will cease to work. Some diesel engines will operate on reduced power mode or limp mode. This precaution limits the number of times you can turn the engine over.

Treat your DEF tank just like your fuel tank — to prevent you from being stranded because of an empty DEF tank. 

3. Where Can I Buy DEF?

You can’t find a DEF supply just anywhere. It’s mostly sold at truck stops in big jugs containing multiple gallons.

Some gas stations have DEF but don’t count on it in emergencies since not all gas stations will stock it. Instead, refill your DEF tank when you can or carry an extra DEF supply in your vehicle for emergencies. 

You can also buy DEF for your diesel vehicle at Walmart or Target. 

4. Can I Substitute Water for DEF?

Absolutely NOT! 

Please avoid pouring water into your DEF reservoir, as your diesel vehicle will identify it as poor quality and won’t start. Even if your vehicle starts — water can’t convert harmful gas into harmless ones like diesel exhaust fluid. So, you’ll be putting your car and the environment at risk

5. How Do I Store DEF, and for How Long?

You should store DEF in a cool, dry, well-ventilated area that’s out of direct sunlight at about 77℉ (25 ℃). 

Also, store DEF in unreactive and non-corrosive containers made from compatible materials, as we’ve mentioned before. Avoid reusing old containers, as they can contaminate the DEF as well. 

Shelf life: 
If you store DEF between 10 and 90 ℉, it could last up to a year. However, if the DEF maximum temperature doesn’t exceed 75℉, its shelf life could extend to approximately two years. 

Always follow the manufacturer’s recommendations for storage and shelf life.

Wrapping Up

Diesel exhaust fluid is essential in diesel engines, especially if you want to comply with emissions regulations and keep your truck running smoothly. If you need help with your DEF tank or require diesel engine repair and maintenance — call AutoNation Mobile Service

AutoNation Mobile Service’s expert mobile mechanics come straight to you! 
Whether you’re driving a diesel pickup or heavy duty vehicle, all our repairs come with a 12-month/ 12,000-mile warranty. 

Contact us today for a consultation using our online booking service.