What is DEF Fluid?

One of the main reasons people choose diesel engine vehicles is they often require less maintenance than gas-powered vehicles. This doesn’t mean they’re maintenance-free, however. You must remember to check a few specific things when you own a diesel vehicle, including the diesel exhaust fluid (DEF).

Let’s go over what DEF is and answer questions like which vehicles need it, what happens if you run out, and how it reacts to freezing temperatures.

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What Is is DEF?

DEF fluid is a nontoxic, non-polluting, non-hazardous, and nonflammable liquid made to reduce the amount of air pollution created by diesel engines. To be specific, it’s an aqueous urea solution consisting of 32.5% urea and 67.5% deionized water. It’s also referred to as AUS 32 (aqueous urea solution 32%) and is sold under the AdBlue trademark. Since “DEF fluid” is redundant (as DEF is an acronym for diesel exhaust fluid), we will be using the term DEF for the remainder of this article.

DEF is injected into a vehicle’s exhaust after it exits the engine, before it’s vented through the tailpipe. This produces a chemical reaction that transforms harmful pollutants into clean air.

DEF is usually clear but can appear light blue when it contacts copper or brass, and light brown if it touches steel or galvanized steel. It’s heavier than diesel fuel and oil and smells slightly of ammonia. DEF should be stored in a cool, dry environment out of direct sunlight.

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Who needs DEF?

DEF must be used with a selective catalytic reduction (SCR) system. An SCR system uses DEF and a catalytic converter to reduce nitrogen oxide (NOx) emissions by up to 90%. SCR has been installed on almost every diesel vehicle made after 2010. In these vehicles, DEF is just as important as any other fluid you regularly check and replace (e.g., fuel, coolant, oil). They won’t run without it.

In a diesel vehicle, DEF has its own tank. You can tell the tanks apart by the colour of the caps. The fuel tank will have a green cap and the DEF tank will have a blue cap. Never put DEF in the fuel tank or vice versa. This can cause major damage to your vehicle.

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What happens if you run out of DEF?

If you drive a diesel vehicle, your dashboard will have a gauge to show you how much DEF you have, much like the fuel gauge. Low DEF levels will activate a warning light and beeping.

Critically low DEF will decrease your vehicle’s performance. If your DEF tank is empty, the engine’s power and speed will be significantly reduced, forcing you to make a pit stop. Some vehicles will refuse to start with no DEF. If your DEF warning light flashes, refill the tank ASAP.

Once DEF is added, some diesel engines will immediately return to regular performance. Others, however, may have reduced performance for days or even weeks. You may also need to have certain parts repaired. For example, your DEF header—a part consisting of level, temperature, and quality sensors—can break when the tank is empty. Running out of DEF multiple times can also cause your vehicle’s exhaust system to clog with soot and debris, making it less efficient and potentially causing it to break down. These repairs can cost hundreds or even thousands of dollars.

When should you refill your DEF, and how much does it cost?

To keep your DEF from getting dangerously low, refill your tank every 4,000 km to 10,000 km depending on your vehicle and load. If you’re worried that you’ll forget, get into the habit of topping up your DEF when you refill your fuel tank.

DEF consumption is usually 1 to 4% of your vehicle’s fuel consumption. In other words, you can expect to use one to four litres of DEF for every 100 litres of diesel fuel.

The national pump price for DEF in Canada is $0.80 per litre. You can also purchase your own supply to refill at home. You can find DEF on the NAPA website and in our stores.

What happens if DEF freezes?

The freezing point of DEF is -11°C. However, cold temperatures don’t affect the efficacy of DEF. Even if the fluid freezes, its quality won’t be affected.

Your engine will still start if the DEF is frozen. Don’t add anything to the DEF: it must remain pure to work correctly. Most tanks will have a heating element to help thaw the fluid.

When driving in low temperatures, keep your DEF tank partially filled. If the tank is full when it freezes, the water inside the DEF will expand and could crack the tank.

Diesel maintenance

Aside from checking your DEF levels, certain maintenance tasks are required to keep your diesel engine vehicle in good condition:

For more questions on DEF or diesel engine vehicles, visit your local NAPA Auto Parts store to speak with an expert.


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