8 Common Questions about Diesel Exhaust Fluid Answered Featured Image

8 Common Questions about Diesel Exhaust Fluid Answered


Diesel Exhaust Fluid, commonly known as DEF, is a crucial component for modern diesel engines. It reduces harmful emissions and ensures compliance with environmental regulations. Let’s cover eight of the most frequently asked questions about DEF.

What is DEF, and how does it work?

Diesel Exhaust Fluid (DEF) is formulated from 32.5% urea and 67.5% deionized water. DEF reduces the emissions of nitrogen oxides (NOx), which are harmful pollutants produced during the combustion of diesel fuel. The process begins when DEF is injected into the exhaust stream of a diesel engine. This occurs before the exhaust gases pass through the vehicle’s Selective Catalytic Reduction (SCR) system. The high temperatures in the exhaust stream cause the DEF to vaporize and decompose into ammonia and carbon dioxide. Once in the SCR catalyst, the ammonia reacts with the NOx gases, converting them into nitrogen and water vapor. These are natural components of the air we breathe and do not harm the environment. This chemical reaction is crucial for maintaining the vehicle’s compliance with environmental regulations by significantly lowering NOx emissions. In conjunction with DEF, SCR technology has become a standard in the automotive industry for ensuring that diesel-powered vehicles operate cleaner and more efficiently.

How often does DEF need to be refilled?

The necessity to refill DEF in your vehicle hinges on several factors, including the vehicle’s design, how it’s driven, and the conditions under which it operates. Typically, a vehicle will use DEF at 2% to 4% of its diesel fuel consumption. For every 100 gallons of diesel fuel consumed, you’ll need between two (2) and four (4) gallons of DEF. For drivers who primarily use their vehicles for long-distance hauls or heavy-duty tasks, this may translate to more frequent DEF refills than those using their vehicles for lighter or shorter trips. The vehicle has a monitoring system will alert the driver when the DEF level is low, generally giving ample time to refill the tank before it runs completely dry. It’s a good practice to check the DEF level regularly, especially before embarking on long journeys, to ensure uninterrupted vehicle performance. The specific interval at which your vehicle will need a DEF refill can also be found in the owner’s manual, providing a more tailored guideline based on the manufacturer’s recommendations.

Is DEF compatible with all diesel engines?

While DEF is primarily designed for modern diesel engines fitted with  Selective Catalytic Reduction (SCR) technology, it’s not universally compatible with all diesel engines. Vehicles and equipment that predate introducing SCR technology will not have the necessary systems to utilize DEF. For those modern diesel engines incorporating SCR systems, DEF is not only compatible, it’s essential for reducing emissions and meeting environmental standards. Owners of older diesel engines or those unsure about their vehicle’s compatibility should consult their vehicle’s documentation or a professional to understand if DEF usage applies to their situation. This ensures that the DEF fulfills its intended purpose without risking potential harm to engines not equipped to handle it.

Can DEF freeze in cold weather conditions?

Indeed, DEF has a freezing point of 12°F (-11°C), which means it can solidify in colder climates. Despite this, vehicle manufacturers have designed DEF tanks and delivery systems to cope with such conditions. When DEF freezes, the integrity of the solution remains unchanged. Upon warming, it will thaw back to its liquid state without degradation in quality or performance. Vehicles equipped with SCR technology are generally designed to operate without issue if the DEF freezes, often featuring built-in heaters that ensure the DEF thaws quickly and maintains fluidity for proper injection into the exhaust stream. It’s advisable for vehicle owners in colder regions to keep an eye on the DEF levels during winter months and to understand that the freezing of DEF does not signify a problem with the product or the vehicle’s emissions system. Proper storage and maintenance practices ensure that DEF remains effective year-round, regardless of temperature fluctuations.

Are there any environmental benefits to using DEF?

Utilizing DEF in diesel engines advances our endeavors to conserve the environment and public health. DEF plays a critical role in mitigating air pollution by effectively reducing the levels of nitrogen oxides (NOx) emitted by diesel vehicles. NOx gases are notorious contributors to smog and acid rain, both of which negatively affect ecosystems, wildlife, and human health. The reduction of these emissions through the use of DEF not only helps meet the strict environmental regulations but also significantly contributes to improving air quality. This, in turn, reduces respiratory ailments and other health concerns linked to low air quality, showcasing the direct impact of DEF usage on enhancing public health. Moreover, the push towards using DEF aligns with global initiatives to combat climate change by minimizing the output of harmful pollutants from combustion engines. Adopting DEF in diesel engines underscores an essential shift towards sustainable and responsible vehicle operation, highlighting a collective move towards a cleaner, healthier planet.

What happens if DEF runs out while driving?

Running out of Diesel Exhaust Fluid (DEF) during a journey can significantly impact your vehicle’s performance due to DEF’s critical role in reducing emissions. Modern diesel engines equipped with Selective Catalytic Reduction (SCR) technology are designed to ensure adherence to strict emission standards. Without adequate DEF, these systems cannot function properly, leading to a default mode of operation initiated by the vehicle’s computer system. This mode, often called “limp mode,” is designed to limit the vehicle’s speed and power to minimize emissions and prevent potential damage to the SCR system. The engine’s performance will be noticeably restricted. In some vehicles, this may trigger warning lights on the dashboard or a notification to alert the driver of the DEF deficiency.

It’s important to note that while the vehicle can still operate in this reduced capacity, the experience is intended to encourage an immediate refill and is unsuitable for prolonged driving. Operating a vehicle without DEF can eventually lead to a complete inability to start the engine once it’s turned off, as part of the vehicle’s programmed compliance measures with emission standards. To avoid this stage, drivers are advised to refill the DEF tank when they receive the initial low-level warning, well before the supply is fully depleted. This proactive approach guarantees the vehicle’s continued operations and compliance with environmental regulations while also safeguarding the longevity and efficiency of the SCR system.

Where can I buy DEF?

Finding DEF for your diesel engine is straightforward and convenient, as it is available at numerous retail locations. Drivers can purchase DEF at automotive supply stores, which typically offer a variety of brands and sizes to suit different needs. Additionally, many gas stations and truck stops have DEF available at the pump, making it easy to fill up while refueling your diesel vehicle. Buying DEF directly from vehicle dealerships is also possible, especially if you’re looking for a product recommended by your vehicle’s manufacturer. When shopping for DEF, it’s essential to look for products that meet the ISO 22241 standard to ensure that they are high quality and suitable for use in your vehicle. This standard guarantees that the DEF will function correctly with your vehicle’s SCR system, helping you maintain optimal performance and emission compliance.

Are there any safety precautions that should be taken when handling DEF?

While Diesel Exhaust Fluid (DEF) is not classified as hazardous, specific safety measures are advisable to minimize potential risks during handling. Protective gloves should be used when handling DEF to prevent skin irritation, as prolonged exposure may cause discomfort. Eye protection should also be considered to avoid irritation from splashes. 

If there is an accidental spillage on the skin or in the eyes, it’s essential to rinse the affected area with plenty of water. Similarly, if DEF is spilled, immediate clean-up is advised to avoid slipping hazards and protect the environment from contamination. Store DEF in its original container or in a container specifically designed for DEF use to ensure the product’s integrity and prevent chemical reactions with unsuitable materials. Ensure that DEF containers remain tightly closed when not in use and store them in a stable location, avoiding extreme cold or heat to preserve their quality. Following these precautions and the manufacturer’s guidelines will ensure DEF’s safe and effective use, contributing to its effectiveness in emission control and engine performance.

For more information on purchasing Diesel Exhaust Fluid for your operation,  contact one of our experts.