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Extended Warranty Requirements Would Be Rescinded

The biggest cost driver in the 2023 rule was not the engine technology itself—it was the dramatically expanded emissions warranty. Under the proposed revision, EPA would rescind the extended warranty provisions and retain the current warranty structure for model year 2027 and later engines. That means maintaining today’s 5-year/100,000-mile emissions warranty rather than moving to the 10-year/450,000-mile warranty for heavy-duty trucks as prescribed in the 2023 rule.

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Longer Useful-Life Requirements Would Be Delayed

The 2023 rule significantly extended the period over which engines must continue meeting emissions requirements. While some manufacturers indicated they were on track to reach those targets, others warned they needed more real-world operating experience. EPA proposes delaying longer useful-life requirements until model year 2030. For fleets, the practical effect is that engines sold between model years 2027 and 2029 would remain subject to today’s useful-life standards rather than the more aggressive requirements scheduled under the original rule. The delay does not change the 2027 NOx standard itself. Instead, it provides additional time for manufacturers to demonstrate durability performance while minimizing the risk of recalls.

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Specialized Vocational Equipment Gets More Flexibility

Many vocational, specialized vehicles—such as fire and garbage trucks—have difficulty meeting emissions requirements. Recognizing that reality, the 2023 NOx rule created a low-volume production allowance permitting manufacturers to certify a limited number of engines to pre-2027 standards. However, access to that flexibility required manufacturers to surrender NOx credits, and several OEMs told EPA they lacked sufficient credit balances to make meaningful use of the provision. EPA proposes removing the requirement for OEMs to use NOx credits to access the allowance for specialized vehicles while preserving other conditions to limit the impact on emissions.

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EPA Would Create a Safety Valve Through Nonconformance Penalties

One of ATA’s key recommendations was the creation of nonconformance penalties (NCPs), a long-standing exemption permitted by the Clean Air Act. EPA proposes allowing diesel medium- and heavy-duty engine manufacturers to utilize NCPs if they cannot achieve the 2027 NOx standard on time. Engines would still have to satisfy emission limits, but manufacturers could temporarily certify products above the 35 mg/hp-hr NOx standard by paying a per-engine penalty tied to their level of noncompliance. For fleets, the significance is straightforward: NCPs reduce the risk that certain engines will disappear from the market. This could help avoid supply chain disruptions and maintain competition among engine manufacturers during the transition to the new standards.

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Driver Warning Systems Would Replace Speed and Power Derates

Perhaps the most operationally significant proposal for fleets involves diesel exhaust fluid (DEF) inducements. When a DEF issue arises, EPA proposes replacing speed and power derates with audible notifications. The audible alert would supplement dashboard warning lights, which are already required. EPA threw its weight behind this proposal following operator testimony about derates causing unsafe roadside stops, towing bills, missed loads, and reputational harm. Additionally, EPA cited evidence that derates are often triggered for reasons unrelated to empty tanks or tampering.

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Numerous Technical and Certification Requirements Would Be Updated

EPA’s proposal includes a series of technical amendments to align the heavy-duty powertrain testing and maintenance procedures with light-duty vehicle procedures to limit the test burden for manufacturers that certify both light- and heavy-duty products. While these revisions may not be as visible as other elements of the proposed rule, they could help reduce compliance complexity and support a smoother rollout of next-generation emissions.