EF, or diesel exhaust fluid, is a critical ingredient in modern diesel engine systems that converts dangerous nitrogen oxide (NOx) emissions to harmless nitrogen and water vapor as part of the selective catalytic reduction (SCR) process. Due to the tightening of emissions standards in the early 2010s, almost all commercial diesel engines and many consumer diesel vehicles require DEF to operate, and failing to maintain a supply of DEF can cause a diesel engine to shut down completely.
Read more to learn more about diesel exhaust fluid, including chemistry and function, regulation history, and best practices for maintaining a diesel system that uses DEF. Then shop Diesel Dash online or call 833-DSL-DASH for DEF and DEF-related products at a great price.
What you will learn:
DEF is not a fuel additive; instead, it is a precise mixture of urea and deionized water that is sprayed into the exhaust gas before entering the diesel engine’s SCR catalyst. Through a chemical reaction, the DEF helps break down the nitrogen oxides (NOx) generated by the exhaust system into environment-friendly nitrogen, water vapor, and tiny amounts of carbon dioxide (CO2).
DEF is a chemical solution containing exactly 67.5% deionized water and 32.5% automotive-grade, aqueous urea. The urea component is an organic compound made of nitrogen, hydrogen, carbon, and oxygen. We know what you’re thinking--yes, technically, urea is one of the components of urine, but for the auto and trucking industry, it is synthetically produced. DEF is a clear, colorless liquid with a similar appearance to water, but it has a faint odor of ammonia.
DEF is a safe, pure, and stable fluid and is regulated by the American Petroleum Institute (API). The EPA governs the production, handling, and transportation through its ISO 22241-1 standard.
DEF is also known as an exhaust fluid under the brand name BlueDEF® and as an aqueous urea solution of 32% (AUS 32).
As a compound, nitrogen turns to ammonia when heated. The aftertreatment system introduces ammonia given up from the urea found in DEF to initiate a chemical reaction to clean up the exhaust system NOx.
DEF goes through three chemical reaction stages in the SCR system:
Simplified, this is what the reaction looks like: NH3 + NOx = N + H2O.
Modern diesel exhaust systems are complex. Components have advanced technology and multiple sub-systems. Some of these include a diesel particulate filter (DPF), an oxidation catalyst, and a reduction catalyst.
So what is catalysis? It is a substance that increases the rate of a chemical reaction without itself going through a chemical change.
Since 2010, all new medium and heavy-duty on-highway diesel engines must comply with the Environmental Protection Agency’s (EPA) regulations to bring engine emissions to near zero levels. To comply with cutting down emission levels, diesel engine manufacturers as a whole decided to implement a selective catalytic reduction (SCR) system, which includes an SCR catalyst, a pump, a tank, lines, an injector, and a synthetic, non-toxic diesel exhaust fluid (DEF). These components work together as part of the after-treatment system to clean the diesel exhaust.
While there are other systems besides SCR that can reduce emissions, such as exhaust gas recirculation or lean NOx trap (LNT), SCR is typically the most effective method for reducing NOx without impacting engine performance, especially for heavy-duty vehicles.
Since 2010, almost all new diesel engines are equipped with an SCR system and therefore require DEF to operate. This includes:
Even commercial marine vessels such as fishing boats and cargo ships require DEF as part of the EPA’s marine emissions regulations rolled out in 2015. Maritime DEF is subject to particularly strict standards for composition and storage.
Here are some advantages of SCR systems and DEF as an aftertreatment system in diesel engines:
Here are some common disadvantages of using SCR and DEF as an aftertreatment system:
Imagine this scene a truck driver has been cruising along for the past 500 miles with a low diesel exhaust fluid (DEF) warning light illuminated on the dashboard. They think to themselves, “It can wait a little longer. Filling up the DEF tank just adds to the cost I’m already paying for diesel. DEF is for emission reductions, right? I might be going against regulations if it runs out, but it’s part of the after-treatment system, not actually part of part of the engine itself. I’m so close to home and I’ll fill it up before my next job—what’s the worst that could happen?” Little do they know that their worst trucking nightmare is about to begin. Soon, your DEF tank will be empty. Depending on your truck’s manufacturer, one or more of the following scenarios will happen.
Eventually, the engine will not start at all with a low fluid level or an empty DEF tank. Don’t be that trucker who gets stranded in the middle of nowhere because you thought you could get away with not filling up the tank. Treat the DEF level as you would the fuel level… when it’s low, fill it up with quality fluid.
Running out of DEF won’t causing any lasting harm to your engine, but laws in the United States, several Canadian provinces, and other regions require that diesel systems shutdown or go into a derated power mode until the DEF tank is refilled. Bypassing these restrictions by tampering with or modifying the SCR system is often illegal and can result in heavy fines.
DEF is stored in an independent, heavy-duty plastic tank that is corrosion-resistant and heated by the coolant lines. The DEF tank will always have a blue filler cap. The tank’s fluid level is monitored by the dashboard DEF gauge.
So how often should you fill the DEF tank? Refill intervals vary based on hours used, miles traveled, operating conditions, and load factors. As part of your pre-inspection, the DEF level should be checked and filled and every time you fuel up, check and fill up the tank if necessary. When the DEF warning lamp goes off, work on fixing the low fluid level ASAP.
How often the DEF tank needs to be topped off depends on your engine and the load you’re hauling. For the most part, the heavier the load, the more the tank needs to be filled up.
Typically, the amount of DEF a diesel system consumes is still a fraction of diesel fuel use: DEF consumption is about 3% of fuel consumption. In other words, for every 100 gallons of fuel a diesel engine uses, the SCR system consumes about 3 gallons of DEF.
Like fuel, the cost of DEF varies in price depending on location, time of the year, and availability. Expect to pay about $11 per gallon for brand-name, high-quality DEF from an online store as of January 2023.
While it can be true that DEF from the truck stop pump is convenient and cheaper, experts warn that there can be risks associated with buying DEF from the pump, such as low-quality fluid and contamination problems, just to name a few key issues. Experts warn that there can be risks associated with buying DEF from the pump—including fluid contamination, evaporation, degradation, and expiration—especially if the fluid sits in the tanks for an extended period of time or is exposed to temperature extremes.
DEF filters need to be regularly inspected, cleaned, and replaced if necessary. Contaminants and temperature changes can clog the filter with dirt, dust, or urea crystals. A clogged filter can cause decreased engine performance, reduced efficiency of the after-treatment system, and even engine shutdown.
If you don’t perform routine maintenance on the DEF system components, three things could happen:
Prevention and proactive monitoring help prevent DEF and DEF-system repairs. While performing routine maintenance can be a pain, having a mechanic service a run-down DEF system is time-consuming and expensive. Refer to your owner’s manual on when to service the DEF system and always have it worked on by a trained professional.
When stored at temperatures between 12ºF and 86ºF (11ºC and 30ºC), DEF typically has a shelf life of 1 year. While it’s always best to use DEF quickly, storing the fluid in a dark, dry location and around 75 ºF (24ºC) can minimize degradation and extend the shelf life.
Always throw out DEF that is discolored, cloudy, or more than three years old.
Most DEF-related problems are from improper handling, contamination, and infrequent replacements. It’s important to handle, store, and replace your DEF correctly. When this fluid is contaminated, your truck’s performance will suffer. If the DEF in the tank is cloudy or hard to see through, it is time to replace the fluid. Refer to your owner’s manual on how to change out DEF and always have your DEF system serviced by a trained professional.
If your DEF is freezing at a temperature above the DEF freeze point of 12 °F, it has been compromised or contaminated in some way. Likely, the DEF system is not properly sealed, or the water in the formula has evaporated, or the urea concentration has increased (what is known as salted-out urea).
When this type of freeze happens in the tank, the DEF must be drained and replaced immediately. Once out of the tank, store the contaminated DEF in an acceptable DEF storage container until it can be recycled. If frozen while still in the bottle, it can be set aside and recycled responsibly. Check with your local and state waste treatment facility on how to properly dispose of bad DEF. Refer to your owner’s manual on how to change out DEF and always have your DEF system serviced by a trained professional.
These red flags shouldn’t be ignored either. Troubleshoot these issues as soon as you can.
Replacing your fluid is one of the easiest ways to prevent increased consumption and possible engine and system damage. Replacing this fluid, however, is complicated. The procedure involves removing the DEF tank. Refer to your owner’s manual on how often to change out DEF and always have your DEF system serviced by a trained professional.
Do not allow these metals and materials to come in contact with DEF:
Will there come a time when DEF will be thought of as the blinker fluid of the 21st century? Probably not. It’s been over a decade since the EPA first implemented strict regulations to clean up Nox in exhaust gas. During this time, the volume of DEF consumed has grown steadily each year along with the distribution system and the number of customers that use DEF. DEF successfully reduces tailpipe emissions levels according to the current EPA standards, so DEF is likely here to stay for the immediate future.
So where can I buy DEF? Like a good scout, always be prepared and keep a jug or two stashed away in your cab or environmentally-controlled home base. The easiest way to purchase DEF is to order it online and have it shipped directly to you. Diesel Dash offers same-day shipping and convenient store pickup from our select network of locations.
Shop Diesel Dash online or call 833-DSL-DASH for DEF and DEF-related products at a great price.
All DEF Products – Diesel Dash
20 Facts You Need To Know About Diesel Exhaust Fluid (DEF) – Capital Reman Exchange
What Does DEF Do? A Guide to Using Diesel Exhaust Fluid – KUS
What’s the Point of DEF Fluid in Your Truck? – Drivin’ & Vibin’
Know the Shelf Life of Diesel Exhaust Fluid – Blue Sky DEF
Recognizing and Preventing DEF Expiration – Retif Oil & Fuel
Storing Diesel Exhaust Fluid (DEF) in the Summer – Fuel Logic
Tech talk #8 Part 2/2: Everything you need to know about DEF fluid – MotorCity Mechanic
Stewart & Stevenson Diesel Service and Repair Locations