The U.S. Environmental Protection Agency stated on February 3 it plans to independently evaluate the next steps it will take in 2026 pertaining to diesel exhaust fluid (DEF) system failures that result in sudden speed losses and shutdowns.
The EPA called on manufacturers to revise DEF system software in existing fleets to prevent these sudden shutdowns and give operators more time to repair faults affecting emissions control and engine performance.
DEF systems are designed to reduce nitrogen oxide (NOx) emissions in diesel engines through a process called selective catalytic reduction (SCR), injecting a urea-based fluid into the exhaust stream. That injection reduces the exhaust to harmless nitrogen and water vapor. The main components of the SCR process are:
- DEF tank
- DEF pump
- DEF injector (doser)
- SCR catalyst
- NOx sensors (pre-SCR and post-SCR)
- Heater elements (inside tank + lines)
- DEF quality sensor.
If any of these components have a problem, the system gives a warning, and if the warning is not taken care of, forces the truck into “limp” mode to protect the aftertreatment system, a condition called a deratement. The system gives warnings before any damage and operational issues may arise but ignoring the warnings can result in trucks going into a “speed limited” mode as low as 5 mph. When the system cannot adjust for a problem, it protects the truck.
The EPA is asking major diesel engine manufacturers to provide critical data about diesel exhaust fluid system failures to remedy equipment problems for operators of these vehicles. The EPA is working with manufacturers to prevent deratements by revising the software to avoid system shutdown and give more time to truck owners/operators to repair the components without impacts to productivity and safety.
The EPA is also working on its proposal for the reconsideration of the 2022 Heavy-Duty Engine and Vehicle NOx rule to determine whether derates may no longer be necessary for air quality compliance.
In addition, the EPA stated that starting with model year 2027, all new diesel on-road trucks must be engineered to avoid sudden and severe power loss after running out of DEF.
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