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89. Diesel exhaust fluid

Diesel exhaust fluid (DEF) is a liquid used by many modern diesel vehicles with selective catalytic reduction (SCR) systems to reduce harmful nitrogen oxide (NOx) emissions. In Europe it is commonly known by the trade name AdBlue.

DEF is not diesel fuel and is stored in a separate tank. It is a solution containing high-purity urea and water that is injected into the exhaust system, where it helps convert nitrogen oxides into less harmful substances.

Diesel exhaust fluid
AdBlue diesel exhaust fluid in 1.5-litre and 10-litre containers.
Wikimedia.org • Lenborje • CC BY-SA
What is diesel exhaust fluid used for?

Diesel exhaust fluid is used in vehicles equipped with an SCR system. It is injected into the exhaust gases to help reduce nitrogen oxide emissions produced by the diesel engine.

Is diesel exhaust fluid the same as diesel fuel?

No. Diesel exhaust fluid is not a fuel and must never be poured into the diesel tank. Vehicles that use it have a separate DEF tank, usually identified by a dedicated filler cap.

What should you do when the DEF warning appears?

Refill the DEF tank with fluid that meets the specification required by the vehicle manufacturer. Do not ignore the warning, because some vehicles can restrict operation or prevent restarting if the DEF supply becomes empty.

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