Diesel Exhaust Fluid 101
What is diesel exhaust fluid and how does it affect your heavy-duty truck sensors and aftertreatment systems?
As the trucking industry prepares for the EPA's more stringent low-NOx emissions standards taking effect in 2027, diesel exhaust fluid (DEF) is expected to play an even larger role in emissions compliance.
While DEF has been a standard component of heavy-duty diesel engines since the 2010 model year, misconceptions about its purpose and operation remain common among fleets and drivers.
Understanding how DEF works and how it interacts with modern aftertreatment systems can help fleets improve reliability, avoid unnecessary downtime and prepare for changes to emissions technology. Josh Tippin, vice president of fuels and energy services for Penske Transportation Solutions, answers some of the frequently asked questions Penske receives on DEF.
What is diesel exhaust fluid, or DEF, and why is it important?
DEF is a fluid made primarily of purified water and urea that is used in selective catalytic reduction (SCR) systems on diesel engines. It is stored in a separate tank and injected into the exhaust stream, where it reacts with a catalyst to convert nitrogen oxide (NOx) emissions into nitrogen and water vapor.
Modern diesel engines are designed to operate with DEF as part of their emissions systems. Without it, the SCR system cannot function properly, which can eventually lead to vehicle derates that reduce engine performance.
DEF has become a standard component of diesel-powered commercial vehicles and plays an essential role in helping fleets meet emissions requirements while maintaining fuel efficiency.
Why are fleets hearing more about DEF consumption today?
Newer engine technologies continue to focus on reducing emissions, and many of the systems are expected to rely more heavily on DEF to achieve those reductions. With the 2027 model year, fleets may see modest increases in DEF consumption compared to previous generations of equipment.
While DEF usage varies based on duty cycle, load and operating conditions, it is important for fleets to monitor consumption trends and ensure drivers understand the importance of maintaining adequate DEF levels. The good news is that DEF is widely available through bulk dispensers and packaged containers, making it relatively easy for fleets and drivers to replenish as needed.
What role do DEF sensors play in the emissions system?
DEF sensors help monitor fluid quality, temperature and tank levels. They provide information to the vehicle's emissions control system to ensure the SCR system is functioning as designed.
The sensors are critical because emissions systems rely on accurate data to verify it is in compliance and operating properly. If a sensor detects a problem or if the sensor itself fails, the vehicle may generate warning lights or fault codes that require attention.
Why have DEF sensors received so much industry attention?
The challenge is that a sensor failure can sometimes create operational issues even when the SCR system itself is functioning properly. During sensor shortages, fleets faced situations where replacement parts were difficult to obtain, increasing concerns about vehicle downtime.
In response, regulators and manufacturers worked together on temporary software solutions that helped keep vehicles operating while maintaining emissions system functionality.
One common misconception was that these software updates disabled emissions controls. In reality, the SCR systems continued operating as designed. The temporary changes primarily addressed how the vehicle responded when certain sensor failures occurred.
What are the most common DEF-related maintenance issues fleets encounter?
DEF contamination remains one of the most common issues. Because DEF is a highly controlled fluid, even small amounts of dirt, fuel or other contaminants can affect system performance.
Improper storage can also create problems. DEF is temperature sensitive and should be stored according to manufacturer recommendations. Exposure to extreme temperatures or contamination can lead to fluid degradation and system issues.
Crystallization of urea can occur when DEF is exposed to air or when leaks develop around injectors or fittings. Over time, these deposits can affect system performance if they are not addressed during routine maintenance.
How does cold weather affect DEF systems?
DEF freezes at approximately 12 degrees Fahrenheit. Trucks are equipped with heaters designed to thaw DEF and keep the system operational, but it is important to verify those heaters are functioning properly before winter weather arrives.
Cold temperatures can also increase the risk of DEF pumping equipment issues, such as frozen DEF lines, pump and injector stress, and crystallization between thaw cycles. Colder temps also introduce moisture-related issues, particularly for sensors, wiring and electrical connections. Road salt and corrosion can accelerate wear on components throughout the aftertreatment system.
Preventive maintenance inspections before winter can help identify potential concerns before they lead to roadside events.
What other aftertreatment components require attention?
The diesel particulate filter (DPF) is another critical component. DPFs capture soot and particulate matter from the exhaust stream and must periodically regenerate to burn off accumulated material.
Vehicles that idle extensively or operate on short routes may not consistently reach the temperatures needed for passive regenerations. When that happens, the system may require forced regenerations, which can take vehicles out of service temporarily.
Over time, DPFs also require scheduled cleaning as part of routine maintenance. Following manufacturer recommendations and monitoring filter performance helps reduce the risk of restrictions and derates.
What can fleets do to reduce aftertreatment-related downtime?
A proactive maintenance strategy is the No. 1 tool to help reduce unscheduled downtime related to the aftertreatment system. That includes using high-quality DEF, maintaining the system, monitoring fault codes and following recommended DPF cleaning intervals.
Driver education is also important. Drivers should understand warning indicators, complete required regenerations when prompted and report issues promptly. Many fleets are also leveraging telematics, diagnostics and predictive maintenance tools to identify potential concerns before they become roadside failures.
What should fleets expect moving forward?
Emissions systems will continue to evolve as manufacturers work to meet stringent environmental requirements, and fleets should expect continued advancements in aftertreatment technologies, sensors and software capabilities.
At the same time, the fundamentals remain unchanged. Proper maintenance, high-quality fluids, routine inspections and timely repairs are still the most effective ways to maximize uptime and ensure emissions systems perform as intended.