What is DEF?

You may have noticed that there is an extra fill spout next to your diesel fuel cap or a new warning light on your dashboard. That is for your DEF fluid (diesel exhaust fluid). DEF fluid is a mixture of 32.5% formaldehyde-free low biuret urea (ammonia) and 67.5% deionized water.

Why is it Important?

DEF is not a fuel additive, because it does not and should not come into contact with your fuel directly. If you accidentally add DEF fluid to your fuel, you could ruin your fuel system! The DEF fluid is an integral part of a selective catalytic reduction system.  It is designed to be added to your truck’s exhaust as a catalyst which initiates a chemical reaction that breaks down the oxides of nitrogen into water and nitrogen. In order to meet the more stringent emissions requirements set by the EPA in 2010, many truck manufacturers have moved to selective catalytic reduction systems.

Don’t Ignore your DEF Fluid!

It may be tempting to ignore adding your DEF fluid because it is unfamiliar or annoying. However, that is not a good idea! Some truck manufacturers have made it so that your truck won’t run if you fully run out of DEF fluid.

When do I Need it?

Most of the time, you will only need to add it a few times per year. Typical DEF consumption is 1.6% to 2% of diesel fuel consumption. Basically, for every 100 gallons of diesel fuel you use, you will need to add 1.6-2 gallons of DEF fluid.

Your truck will warn you when you are getting low. Your in-dash display will either show a written warning like “DEF Low” or look for a warning light that looks like one of these:

How do I add it?

Adding DEF fluid is easier than you might think! It is available at most truck stops and auto parts stores. You don’t need any special equipment or protective gear, because DEF is nontoxic to handle (just don’t drink it!). The most commonly available DEF fluid is sold under the brand name AdBlue, but there are many other brands entering the market.

While the owner’s manual will be your best resource on how to add DEF fluid, generally you will find the fill spout next to your diesel fuel spout or under the hood of your truck. In some cars, the DEF fill spout is actually in the trunk, where you would expect your spare tire to be! Look for a small fill spout with a blue cap! It will usually be labeled with DEF and/or AdBlue, but not always. Most vehicle manufacturers require that you add at least 2 gallons of DEF fluid when it is time to refill, but each vehicle is a little different on this.

Still need help with your DEF fluid, call our experienced technicians at the Country Truck Service Center! 



Written by:

Beckie Bean

Digital Content Coordinator at Country Truck & Auto

September 22, 2016

Sources: TruckTrend.com, DieselForum.org, and DiscoverDEF.com

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