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.

This is for your DEF (Diesel Exhaust Fluid). DEF is mixture of 32.5% formaldehyde-free low biuret urea (ammonia) and 67.5 % de-ionized water.

Why is DEF 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 to your fuel, you could ruin your fuel system! DEF is an integral part of a SCR (selective catalytic reduction) system. It is designed to be added to your vehicles exhaust as a catalyst which initiates a chemical reaction that breaks down the oxides of nitrogen into water and nitrogen. In order to meet more stringent emission requirements set by the EPA in 2010, many vehicle manufacturers have moved to SCR systems.

Do NOT ignore your DEF!
It may be tempting to ignore adding DEF because you are unfamiliar or it is annoying. However, that is not a good idea! Some vehicle manufacturers designed your vehicle to stop running if you fully run out of DEF.

When does my truck need DEF?
Most of the time, you will only need to add it a few times per year. Typically 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.

Your vehicle will warn you when you are getting low. Your in-dash display will either show a written warning “DEF Low” or look for a warning light that is similar to this one the images below.

How do I add DEF?
Adding DEF is easier than you may think! It is available at most truck stops and auto part stores. You don’t need special equipment or protective gear, because DEF is non-toxic to handle (just don’t drink it!). The most commonly available DEF 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, generally you will find the spout next to your diesel fuel spout or under the hood of your vehicle. In some vehicles, the DEF fill spout is actually in the trunk, where you would expect your spare tire to be! Look for a fill spout with a blue cap. It will be labeled with DEF and/or AdBlue, but not always. Every vehicle is a little different but most vehicle manufacturers require you to add at least two gallons of DEF when it is time to refill.