What Is Diesel Exhaust Fluid (DEF) in Trucking?
Also called: DEF, SCR Fluid, DEF Derate
Diesel exhaust fluid, or DEF, is a liquid that SCR systems spray into a truck's exhaust to cut nitrogen oxide emissions. Since 2010, nearly all on-road diesel trucks use it. If DEF runs out or a sensor fails, the truck can derate, which can cut speed as low as 5 miles per hour. EPA issued guidance in 2025 and 2026 to reduce these derates.
Last reviewed September 23, 2026 · Facts checked October 16, 2026
Key Points
- DEF goes in its own tank, not in the fuel tank. SCR systems use it to reduce nitrogen oxide emissions
- Since 2010, nearly all on-road diesel trucks and many types of nonroad equipment have used DEF (EPA)
- If DEF runs out, or the system's sensors fail, the vehicle can be forced to cut speed sharply or become inoperable. EPA says the derate can be as low as 5 miles per hour
- EPA requires on-board diagnostics to watch the SCR system for low DEF and system failures, and to warn the driver with a light. The system must also be built to prevent tampering
- In August 2025 EPA issued guidance to manufacturers on changing DEF systems to reduce derates, and in March 2026 it issued guidance on using alternate sensors
- By law EPA cannot require field fixes for trucks already in use, so the updates are voluntary for manufacturers. Ask your dealer which updates exist for your truck
- In July 2026 EPA proposed a rule for model year 2027 and later heavy-duty engines that would amend the inducement rules. It was a proposal, not a final rule, when we checked
What is DEF and what does it do?
DEF is a fluid used by selective catalytic reduction, or SCR, systems. The truck sprays it into the exhaust, where it helps turn nitrogen oxides, a major air pollutant, into harmless gases. EPA says that since 2010, nearly all on-road diesel trucks use DEF. It is stored in a separate tank, with its own cap. It is not a fuel additive and it does not go into the diesel tank. Each truck has a DEF gauge or a warning light.
What happens if DEF runs out or a sensor fails?
The truck warns you first, then it can limit its own power and speed. This is called a derate. EPA says the derate strategy can limit a vehicle to nearly inoperable levels, as low as 5 miles per hour. It can be triggered by running out of DEF, by poor DEF quality, or by a failed sensor. EPA says the strategy was meant to ensure that trucks meet emission standards. But it has also caused delays and hardship for truckers. When a warning comes on, fill the tank or get service promptly.
What has EPA changed about derates?
EPA has taken three steps. In August 2025 it issued guidance telling manufacturers how they may change DEF systems to reduce derates. In March 2026 it issued guidance on using alternate sensor technology to check DEF quality. And in July 2026 it proposed a rule for model year 2027 and later heavy-duty engines that would amend the inducement provisions. EPA cannot order changes for trucks already on the road, so the software updates depend on manufacturers. Ask your dealer or engine maker what is available for your engine. Check EPA's page for the status of the proposed rule.
Is DEF going away?
No. EPA is not removing the DEF requirement. Its actions aim to reduce the harsh derates, not to end the use of SCR. EPA still requires vehicles to meet the NOx standards, and the systems must monitor DEF levels, detect failures and resist tampering. Even if the proposed rule changes how derates work for future engines, a truck with an SCR system still needs DEF. Do not remove or bypass any part of the DEF system. EPA says the system must be built to prevent tampering and bypassing.
How do you keep from running out of DEF?
Watch the DEF gauge, as you do your fuel gauge. Many drivers top off DEF when they fuel. EPA says DEF has been made broadly available at truck stops over the past 15 years, but gaps remain, so plan for long stretches without services. Check your truck's manual for the tank size, the type of DEF to use and how to fill it. Store DEF as the maker advises. It is mostly water, so it can freeze in very cold weather. Many systems have heaters for this, but check your manual.
Related Terms
Related Free Tools
DEF Cost Calculator
See how much diesel exhaust fluid your truck uses and what it costs per mile and per year, and work out your own DEF rate from your fill-ups.
Truck Breakdown Triage
Pick what is going wrong and get a clear answer: stop now, get help first, or drive on to the next safe stop. Then copy a message for your broker.
Preventive Maintenance Scheduler
Track all service intervals by odometer. Never miss a maintenance item again.
Diesel Fault Code Lookup (SPN/FMI)
Look up common J1939 SPN and FMI codes — what they mean and what commonly causes them.
Frequently Asked Questions
What is DEF in a truck?+
DEF, or diesel exhaust fluid, is a liquid that trucks with SCR systems spray into the exhaust to cut nitrogen oxide emissions. It goes in its own tank. EPA says nearly all on-road diesel trucks since 2010 use it.
What happens if you run out of DEF?+
The truck warns you, then it can derate, which limits power and speed. EPA says a derate can limit a vehicle to nearly inoperable levels, as low as 5 miles per hour. Fill the DEF tank or get service as soon as a warning appears.
Are they getting rid of DEF?+
No. EPA is working to reduce the harsh derates caused by DEF system failures, through guidance in 2025 and 2026 and a proposed rule in July 2026. But SCR systems still need DEF, and trucks must still meet the NOx standards.
Can DEF freeze?+
DEF is mostly water, so it can freeze in very cold weather. Many systems have heaters for this. Check your manual for how your system works and how to store DEF.
How much DEF does a truck use?+
It depends on the engine, the load and how you drive. Check the manual for your tank size, and keep an eye on the gauge. Many drivers top off DEF when they fuel.
Can I delete or bypass my DEF system?+
No. EPA says the DEF system must be designed to prevent tampering and bypassing. If the system is giving you trouble, ask your dealer about the updates EPA has encouraged manufacturers to offer.
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This is industry terminology and typical practice. It is not legal or tax advice. Rates, fees, and rules mentioned are general ranges. Confirm the details on your rate confirmation, carrier agreement, or a current official source.