Amendments and Nonconformance Penalties for Model Year 2027 and Later Heavy-Duty Highway Engines and Amendments to Inducement Provisions for SCR-Equipped Diesel Engines
Published Date: 7/14/2026
Proposed Rule
Summary
Starting with model year 2027, the EPA is updating rules for big truck engines to make sure they last longer and pollute less. They’re also letting engine makers pay penalties if their engines don’t meet standards, and tightening rules for special pollution-control systems on diesel engines. These changes affect truck and engine makers and kick in soon, with public feedback due by August 29, 2026.
Analyzed Economic Effects
6 provisions identified: 6 benefits, 0 costs, 0 mixed.
Warranty Periods Rolled Back to MY2026
The EPA proposes to keep the emission-related warranty periods for model year (MY) 2027 and later heavy-duty engines at the shorter MY2026 values instead of the longer periods adopted in 2023. The EPA estimates purchasers could save up to $37 billion in emission-related warranty savings overall, including up to $6,000 per diesel vehicle, and net purchaser savings could be as high as $12 billion.
Large Long-Term NOX Reductions Retained
The EPA estimates that, with these proposed amendments, the amended 2023 program would still reduce onroad heavy-duty NOX emissions in 2055 by about 42 percent—over 260,000 tons—compared to the 2055 NOX inventory without the 2023 Final Rule, and that the proposal retains nearly 90 percent of the NOX reductions originally projected from the 2023 Final Rule.
Delay Longer Useful Life To Model Year 2030
The EPA proposes to delay application of the longer regulatory useful life periods (adopted in 2023) so that the longer useful life would begin in MY2030, and the MY2026 (shorter) useful life periods would continue to apply through MY2029. This is intended to give manufacturers more time and data to demonstrate in-use durability over longer periods.
Nonconformance Penalties Available for Some Diesel Engines
The EPA proposes to establish nonconformance penalties (NCPs) under Clean Air Act section 206(g) for diesel-fueled Medium Heavy-Duty Engines (Medium HDE) and Heavy Heavy-Duty Engines (Heavy HDE) beginning in MY2027. NCPs would allow affected manufacturers to produce engines that do not meet the MY2027 NOX standards if they pay the applicable penalty under EPA regulations.
Replace SCR Derates With Alerts for New Diesel Engines
The EPA proposes to replace engine performance derates (power reductions) that were mandated as inducements for selective catalytic reduction (SCR) systems with visible and/or audible notifications for newly manufactured highway engines and vehicles (light-, medium-, and heavy-duty) and for nonroad diesel engines. The Agency is also considering inducement guidance for in-use engines and vehicles.
Keep 5% Production Flexibility; Drop NOX Credit Requirement
The EPA proposes to continue the existing allowance for manufacturers to produce up to five percent of their U.S.-directed production volume of Heavy HDE that are certified to pre-2027 requirements, and to remove the current requirement that manufacturers use NOX emission credits to exercise that allowance.
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Key Dates
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