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What Is Diesel Exhaust Fluid? Understanding Modern Diesel Vehicles

DEF fluid

You’ve probably seen diesel exhaust fluid on the shelf at your local gas station or auto parts store, and wondered what it was for. Or if you own a modern diesel car or truck, you may have even had your dealer tell you that this fluid (also known as DEF) was something you would have to replace on a regular basis.

So how does diesel exhaust fluid work? It’s time to get educated about this emissions-related technology.

No More Dirty Diesels

Once upon a time, diesel engines were notorious for spewing clouds of black soot as they barreled down the highway. This was especially true of 18-wheel transport trucks and heavy-duty pickups, but even European imports from Mercedes-Benz and Volkswagen weren’t exempt from leaving a toxic trail in their wake.

Eventually the EPA decided that enough was enough, and automakers were forced to step up their emissions controls on diesel engines. This meant looking past traditional systems and developing the next-generation of diesel exhaust technology.

Cleaning Up the TailpipeDEF storage tank

Enter DEF. While in the past diesel vehicles had relied on traditional catalytic converters — much like gasoline models — to scrub their tailpipe emissions, the amount of particulate matter produced by diesel motors was too much to handle using a passive design. The decision was made to develop a new type of system that would inject a chemical called “urea” into the catalytic converter in order to assist in the conversion of harmful diesel exhaust into cleaner gases. Urea fluid was given the name diesel exhaust fluid, and it quickly became a popular emissions control method for automakers looking to produce ever more powerful turbo diesel engines.

Stay Topped Up

Diesel exhaust fluid isn’t hard to find, but you still have to remember to keep your tank topped up if your turbo diesel automobile requires it. You’ll usually find the spout with a bright blue cap just beside the gas cap under your filler door, and your vehicle’s manual will tell you how many miles you can typically get out of a single tank of DEF. Your car or truck should provide you with a warning when you are running low, and it’s really not something you should ignore: once your diesel exhaust fluid tank is completely dry, the majority of turbo diesel vehicles won’t start, or will operate exclusively in a “limp” mode at a very low speed until it’s filled again.

Check out all the chemical products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on diesel exhaust fluid, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.

Benjamin Hunting View All

Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time.  I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

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