EPA Proposes Changes to Diesel Exhaust Fluid Requirements
The Environmental Protection Agency has proposed significant revisions to the operational requirements for diesel engines equipped with selective catalytic reduction (SCR) systems. Most notably, EPA would largely replace mandatory engine-power and vehicle-speed derates triggered by diesel exhaust fluid (DEF) issues with a system relying primarily on audible and visual driver warnings.
Agency Looks to Ease Heavy-Duty Truck Pollution Rules
The proposed DEF provisions are part of a broader rulemaking covering heavy-duty diesel engines beginning with model year 2027. The proposal also addresses warranty obligations, useful-life requirements, emissions testing and diagnostics, nonconformance penalties, and other compliance provisions for highway and nonroad engines.
EPA officials said the revisions would reduce compliance costs by 50% and give manufacturers more flexibility. However, the agency is not proposing changes to the underlying model year 2027 nitrogen oxide (NOx) or particulate matter emissions standards.
Driver Warnings Would Replace Most Mandatory Derates
Under current regulations, SCR-equipped engines generally reduce engine power or vehicle speed when they detect low-DEF levels, poor-quality DEF, or possible emissions-system tampering. EPA acknowledges that these inducement strategies have sometimes caused operational disruptions when sensor failures, wiring problems, or diagnostic errors trigger derates even though compliant DEF has been used and no tampering has occurred.
EPA proposes replacing mandatory derates with escalating driver notifications. For low-DEF levels, drivers would receive repeated audible and visual warnings beginning several hours before the DEF tank is depleted and continuing after the tank becomes empty. Separate warnings would apply when the engine detects off-specification DEF or possible tampering.
Manufacturers could still impose engine derates when necessary to protect the engine or aftertreatment system from damage, but any optional vehicle-speed reductions would have to occur gradually rather than immediately.
Proposal Allows Greater Variation in DEF Composition
The proposal would provide greater tolerance for normal variations in DEF composition. In most cases, engines would not be required to issue a DEF-quality warning unless the urea concentration falls below 20 percent. Standard automotive-grade DEF contains 32.5 percent urea. The warning-based approach would become mandatory no later than model year 2029, although manufacturers could adopt it earlier.
DEF Demand Expected to Remain Stable
For ILMA members that manufacture, blend, package or distribute DEF, the proposal is unlikely to materially reduce long-term DEF consumption because SCR-equipped vehicles would still require properly formulated DEF to comply with federal emissions requirements.
The proposal could, however, affect purchasing behavior. If fewer vehicles experience unexpected derates, demand for emergency purchases of packaged DEF may decline with routine replenishment through bulk delivery, pump dispensing or scheduled distribution channels, possibly accounting for a larger share of the market.
Industry Groups Raise Concerns
Not all stakeholders support EPA’s approach. Some industry organizations argue that eliminating mandatory derates could weaken incentives to maintain adequate DEF supplies, create consumer confusion, and negatively affect DEF availability and pricing. They have urged the EPA to allow recently issued compliance guidance and updated diagnostic strategies to be fully implemented before adopting more fundamental
regulatory changes.
Comments on the proposal are due August 29, 2026, under Docket No. EPA-HQ-OAR-2026-0728. EPA has also scheduled virtual public hearings for July 29 and 30, 2026.
