What Is Diesel Exhaust Fluid (DEF)?

When strict diesel emissions standards first hit major truck manufacturers in 2008, the industry faced some severe growing pains. Early systems relied heavily on exhaust gas recirculation (EGR) and diesel particulate filters (DPFs), which led to frequent clogging, fuel-guzzling regenerations, and costly repairs/replacements for owners. Many diesel enthusiasts remember those initial years as the era of skyrocketing maintenance bills and troublesome reliability qualms. 

Fortunately, these issues were largely resolved with the next generation of emissions technology—namely, the introduction of selective catalytic reduction (SCR) and diesel exhaust fluid (DEF). These systems enabled manufacturers to meet stringent emissions regulations while simultaneously improving performance, fuel economy, and long-term reliability. 

In this article, we’ll break down what diesel exhaust fluid is, how it works within an SCR system, what it’s made out of, and why it’s used by nearly every modern diesel truck on the road—from light-duty pickups to massive semis.

To better understand DEF, it’s best to look at the entire SCR (selective catalytic reduction) system. SCR is an emissions system used in modern diesel engines to reduce nitrogen oxide (NOx) emissions—a harmful pollutant produced during a diesel’s high-temp combustion events. 

To explain how the SCR system works, we’ll avoid technical terms and chemical formulas. Here’s an abridged, three-step version of what’s happening in your diesel truck’s tailpipe: 

Step 1: Exhaust gases move from the diesel particulate filter to the SCR catalyst. 

Step 2: Small, precise amounts of DEF are injected into the exhaust stream.

Step 3: Inside the SCR, a chemical reaction converts the harmful NOx into two harmless products—nitrogen and water vapor. 

This system drastically reduces diesel emissions, enabling engines to run more efficiently, pollute less, and meet federal standards.

You thought we’d let you go without a little chemistry? Let’s dive a bit deeper into what actually happens inside the catalyst—don’t worry, we’ll keep things simple. In short, DEF powers the entire SCR system. The fluid enables the chemical reaction within the catalyst that neutralizes nitrogen oxide emissions. 

When DEF is injected into the hot exhaust, it instantly evaporates and breaks down into ammonia (NH₃). The resulting ammonia binds to the NOx in the SCR catalyst, converting it into harmless nitrogen and water vapor.

Even though some drivers see DEF as a nuisance, this emissions system delivers several significant advantages. Check out some of the most notable below, ranging from reduced emissions to improved efficiency. 

The first and most obvious benefit of DEF is its ability to reduce emissions. Diesel exhaust fluid enables the SCR to function, drastically decreasing nitrogen oxide emissions to meet EPA standards. 

SCR systems allow diesel engines to run hotter and leaner without exceeding emissions limits, which improves combustion efficiency. With lower fuel consumption, fuel efficiency increases significantly compared to dated pre-DEF trucks that still feature EGR and DPF systems.

Despite DEF’s benefits, critics still cite various drawbacks.

Ongoing DEF refills can add to fuel-up costs, increasing operating expenses. Additionally, if problems arise in the DEF or SCR system, such as a faulty DEF injector, maintenance and repair costs can be significant.

While DEF is commonly available at various locations around the country, rural areas may have fewer purchasing options. The few and far between DEF sellers will likely raise prices out of necessity.

You’ve likely heard the rumors that DEF is essentially blue urine. While somewhat accurate from a chemistry perspective, the honest answer is: no, diesel exhaust fluid is not derived from living creatures. While DEF does contain 32.5 percent urea (a compound found in urine), the urea used to produce diesel exhaust fluid is entirely synthetically produced to ensure maximum purity. The other 67.5 percent is deionized water. 

If DEF isn’t manufactured to strict ISO regulations, damage to the SCR components may occur.

It was only in 2011–2013 that light-duty diesels adopted SCR and DEF tech, and in 2010 for commercial diesels. However, that wasn’t the first year of diesel emissions systems. For a couple of years prior to SCR and DEF mandates (2008–2010), diesel trucks were fitted with simple NOx, EGR, and DPF systems.

No, not all diesels require DEF. However, all new diesels do. DEF and SCR systems weren’t mandated until 2010 for commercial applications and 2011–2013 for light-duty. Any diesel trucks produced before these regulations weren’t fitted with these systems. 

DEF does degrade, though it can last for quite some time when stored properly. Expect sealed, properly stored DEF to last approximately one to two years or 3,000 to 10,000 miles of driving per full DEF tank, depending on tank size and DEF usage rate.

Due to its high water content, DEF does freeze—however, not until around 12ºF (-11ºC). Fortunately, frozen DEF doesn’t degrade. Plus, most SCR systems include built-in DEF heaters to thaw frozen fluid. What about excessive heat, though? 

Unlike freezing, storing DEF in a hot area or in direct sunlight can significantly degrade the fluid. When possible, store extra DEF in a shady garage or a climate-controlled area to ensure maximum longevity and purity when it’s time to use.

Another commonly asked question piggybacking on storage is whether DEF is flammable. Fortunately, this liquid isn’t flammable, combustible, explosive, or toxic. Being that it’s mostly water, diesel exhaust fluid poses minimal risk when stored correctly.

Until recently, running out of DEF would trigger an immediate power derate and limit operating time, forcing drivers to power down their vehicles and seek repairs/refills immediately. While intended to maximize efficiency and reduce pollutants, this software was recently found to be detrimental to the livelihoods and safety of diesel drivers. In 2025, the EPA allowed vehicle manufacturers to issue software updates that change how diesel trucks handle low-DEF conditions, ditching sudden engine shutdowns for a gradual derating process that still keeps the vehicle functional.

Pricing is a significant concern with DEF, primarily because of the additive's added cost, which must be purchased and used for the vehicle to function correctly. While DEF usage varies based on factors like engine size, driving conditions, and whether or not you’re towing/hauling, most light-duty diesel trucks use DEF at a rate of two to three percent of fuel consumption. In other words, if you use 100 gallons of diesel, expect to burn around two to three gallons of DEF. 

DEF varies in price depending on location, brand, and whether you’re buying individual jugs or refilling at a fuel station. However, typical prices range from $3.80 to $8.00 per gallon. 

DEF is available at various locations, including:

• Auto parts stores

• Big-box stores

• Truck stops

• Fuel stations

• Dealerships

When shopping for DEF, avoid cheap no-name products from disreputable dealers. After all, name-brand DEF is a lot more affordable than the repair bill for a new SCR system.

DEF lasts a considerable amount of time if properly stored. However, should the liquid become contaminated or degraded, you should avoid adding to your truck’s DEF tank immediately to prevent SCR issues. Watch for any of the following symptoms: 

• Unusual color

• Cloudiness or visible sediment

• Strong ammonia odor

• Crystallization around the cap

• SCR-related fault codes

Bad DEF can wreak havoc on the SCR system. When in doubt, dispose of the old fluid and buy fresh DEF.

Now that you’re a DEF expert, it’s time to broaden your understanding of other elements of diesel ownership. To do so, check out our diesel articles on RealSource—your comprehensive source for all things truck, Jeep®, and off-road. 

• Off-Road (Dyed) vs. Regular Diesel 

• The 10 Most Reliable Diesel Trucks in North America

• The Effects of Running Gas Through a Diesel Engine

• Diesel vs. Gas Trucks

• Should You Pick a Diesel in 2025?

• The History of the Cummins Diesel Engine

• The History of the Duramax Diesel Engine