EPA DEF Proposal Moves Past Comments: What Truck Fleets Watch

EPA’s DEF proposal has entered its next stage

The federal comment period has closed on an Environmental Protection Agency proposal that could significantly change how newly manufactured diesel trucks respond to certain Diesel Exhaust Fluid, or DEF, system problems.

As of September 7, EPA is reviewing comments on its proposed amendments covering model-year 2027 and later heavy-duty highway engines. The proposal would replace required engine derates and vehicle speed restrictions tied to selective catalytic reduction, or SCR, inducement systems with audible and/or visible alerts on newly manufactured vehicles. It also proposes changes to emissions-related warranty periods, useful-life requirements and manufacturer compliance options.

The comment deadline was August 29. That means the proposal is not yet a final rule, and fleets should not treat the suggested changes as equipment specifications or operating rules that are already in effect.

For trucking, the issue matters because DEF and SCR faults can create expensive downtime, missed appointments and safety concerns when a truck loses power or speed unexpectedly. At the same time, any change in emissions-warranty and durability requirements could affect the purchase-price, maintenance-risk and resale calculations behind 2027 truck orders.

What EPA has proposed

EPA published the proposed rule in the Federal Register on July 14 under Docket No. EPA-HQ-OAR-2026-0728. It applies to model-year 2027 and later heavy-duty highway engines, along with certain other diesel engines and equipment.

The proposal has four main parts that truck owners and fleets should understand.

1. Replacing new-truck derates with alerts

SCR systems use DEF to help control nitrogen-oxide, or NOx, emissions. A truck may reduce available engine power or vehicle speed when its system identifies conditions involving DEF level, DEF quality or tampering. This power reduction is commonly called a derate.

EPA proposes replacing those derates and speed restrictions for newly manufactured highway diesel engines and vehicles with visual and/or audible notifications. The agency says the proposed approach would allow an operator to receive warnings and continue operating until the issue can be addressed safely.

That point is important: the proposal concerns newly manufactured equipment. EPA also asked for input on whether it should issue updated guidance allowing manufacturers to modify in-use engines and vehicles. No fleet should assume its current trucks will automatically receive a software update or that existing warning, DEF and repair procedures have changed.

2. Revising emissions-warranty provisions

The proposal would modify emissions-related warranty requirements that were scheduled to become more extensive under the 2023 heavy-duty engine rule. EPA’s July fact sheet says the agency proposes maintaining model-year 2026 warranty periods for model-year 2027 and later engines rather than moving ahead with the longer periods previously planned.

For fleets, this creates a practical buying question rather than an immediate answer. A lower upfront truck price does not automatically mean a lower total cost of ownership. The final rule, any manufacturer warranty terms, component coverage, deductibles, roadside support and expected downtime all will matter.

A fleet evaluating 2027 equipment should distinguish between:

• The minimum emissions-related warranty required by regulation.
• The warranty offered by the truck or engine manufacturer.
• Optional extended coverage purchased with the truck.
• Maintenance agreements and their exclusions.
• Costs created by tow events, missed loads, substitute tractors and driver delay.

3. Delaying longer useful-life requirements

EPA also proposes delaying longer regulatory useful-life requirements for heavy-duty engines until model year 2030. The agency’s proposal describes useful life as a regulatory durability period used in emissions compliance.

Useful life is not the same as a truck’s expected service life or a carrier’s replacement cycle. But it can affect engine design, certification testing, warranty planning and the components manufacturers select for their equipment.

For heavy heavy-duty engines, the proposed rule retains a 435,000-mile useful-life framework for model years 2027 through 2029, with a 650,000-mile framework beginning in model year 2030. Those details are highly technical, but they are relevant to carriers that are deciding whether to pre-buy, delay orders or standardize specifications across multiple model years.

4. Creating a nonconformance-penalty option

EPA also proposes making nonconformance penalties available beginning in model year 2027 for certain medium and heavy heavy-duty diesel engines. In general, such penalties would provide a path for manufacturers to continue selling some engines that do not fully meet a standard while development work continues, provided the manufacturer pays a penalty.

EPA says this option is intended to help avoid supply disruptions during the transition to the 2027 standards. Whether this provision is retained in a final rule could matter to truck buyers watching allocation, delivery timing and engine availability.

Why the comment deadline matters

The August 29 deadline closed the main public-comment phase, but it did not settle the debate. EPA must review the record before issuing a final action.

Industry reporting published after the comment period described sharply different views from commenters. Supporters of changing derates have emphasized the operational and safety consequences of unexpected speed loss. Others have raised concerns about preserving incentives to keep DEF tanks full, avoid tampering and maintain SCR systems. Commenters also differed over emissions warranties and useful-life requirements.

That range of views makes the next EPA action especially important. The final rule could retain the proposal, revise it, delay elements or take a different approach based on the administrative record.

For now, the central compliance message is straightforward: DEF remains part of the emissions-control system, and the proposed rule does not authorize disabling, bypassing or tampering with SCR equipment.

Who is affected

Drivers and owner-operators

Drivers may see the biggest day-to-day effect if a final rule changes how new trucks respond to DEF-system faults. A warning-based system could reduce the chance of an abrupt loss of performance, but it would not remove the need to respond promptly to dashboard alerts, inspect equipment and follow manufacturer instructions.

Owner-operators shopping for a 2027 tractor should request complete written warranty information before signing. Ask which SCR, DEF, sensor, dosing, catalyst and control-module components are covered, for how long, and what actions could affect coverage.

Motor carriers and maintenance teams

Carriers should avoid changing preventive-maintenance policies based on a proposal. Instead, they can use this period to inventory their exposure:

• Identify which tractors are current model-year equipment and which are planned 2027 purchases.
• Review recurring DEF and SCR fault codes, roadside events and repair turnaround time.
• Confirm that dispatch, maintenance and drivers use consistent escalation procedures for DEF warnings.
• Track manufacturer bulletins and dealer communications separately from regulatory announcements.
• Compare warranty, uptime and service-network commitments in pending truck quotes.

Recruiters and driver managers

Recruiters and driver managers are not responsible for emissions certification, but truck reliability affects retention. Repeated derates, extended shop stays and unclear breakdown support can reduce driver confidence in assigned equipment.

When discussing late-model equipment with candidates or current drivers, it is better to describe actual fleet specifications and maintenance support than to promise regulatory changes that have not been finalized.

What to watch next

The next major event will be EPA’s final decision or another formal action in the docket. Fleets should watch for four items:

1. **Final rule language.** The exact regulatory text—not summaries or headlines—will determine what changes apply to new trucks. 2. **Effective dates.** A final action may set implementation dates that differ from the proposal’s assumptions. 3. **Manufacturer product announcements.** Engine and truck builders may explain how their 2027 systems, alerts, warranties and certification plans will work. 4. **In-use equipment guidance.** EPA has indicated it is considering whether updated guidance could support changes for equipment already in service.

The proposal offers the possibility of a different approach to DEF-related loss of power on future trucks, but it also puts more attention on warranty coverage, diagnostics and maintenance planning. Until EPA completes the rulemaking, fleets and drivers should continue following the requirements and manufacturer procedures that apply to the equipment they operate today.