How DEF Fluids Work

DEF Fluids are diesel exhaust fluids, which are necessary for keeping your vehicle running healthily. Specifically, it is a mixture of water and urea that is put into your exhaust which then exits through the tailpipe. This process turns potentially dangerous pollutants into clean air.

Video Source

The DEF fluid as seen in the clip attached reacts through a filter, a DEF dosing valve, and an SCR. SCR stands for selective catalytic reduction, which is a high-tech emissions control system that is purposed for reducing tailpipe emissions. Exhaust enters a filter over a catalyst into a wall flow filter where carbon particles are contained in the filter. Nitric oxide in the exhaust gas is then converted into nitrogen oxide in the diesel oxidation catalyst as the nitrogen oxide reacts with the carbon to produce carbon dioxide out of the filter. So it is then water vapor that emits from tailpipes rather than any harmful pollutants. Emissions are reduced to near zero throughout this process in which DEF fluids are involved. If you owned a BMW between 2009 and 2020 you could be owed thousands in compensation due to the diesel emissions scandal.

If you are looking to learn more helpful information on what DEF fluids are and what they’re used for, check out the video we have linked above.


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