The U.S. Environmental Protection Agency (EPA) on Thursday released a long-awaited Notice of Proposed Rulemaking (NPRM) that modifies the heavy-duty truck regulations known informally as the “NoX rule.” According to the EPA, the revised standards will keep the existing nitrogen oxide (NoX) emission limits first promulgated in the Biden administration, but make significant changes to warranty requirements, deratement mechanisms, useful life provisions, and introduce noncompliance penalties. The agency declared the revised standards “will not bring any changes to the cost of truck manufacturing.”
The 477-page NPRM, expected to be published in the Federal Register on Friday, is subject to a 45-day public comment period. The Engine Manufacturers Association, a key trade group, stated it “looks forward to reviewing the proposal in greater detail and continuing to work constructively with EPA to ensure the final rule supports regulatory certainty and practical implementation for manufacturers and our customers.”
What Changed: Four Key Revisions
The proposed revisions fall into four categories, each modifying the 2023 Final Rule:
- Warranty Mileage Reduction: The original rule had increased warranty requirements; the NPRM reduces the maximum warranty mileage from 450,000 miles to 100,000 miles and delays implementation to model year (MY) 2030, rather than 2027 when the broader rule takes effect.
- Elimination of Deratements: The NPRM eliminates engine deratements—where an engine suddenly slows or stops if its emissions system signals a problem—and replaces them with a visual or audible warning that the truck’s emission systems (including lack of diesel exhaust fluid) are failing. Deratements are part of what the EPA calls “inducements” to ensure proper use of selective catalyst reduction (SCR) systems.
- Lengthened Useful Life Provisions: The NPRM delays tighter useful life standards until 2030, allowing older trucks to operate longer than under the original rule. Useful life sets guidelines for how long an older truck can stay on the road.
- Noncompliance Penalties (NCPs): The rule establishes NCPs that would allow an engine manufacturer to pay a financial penalty for failing to meet NoX standards, rather than being forced to stop production.
| Provision | Original 2023 Final Rule | Proposed Change |
|---|---|---|
| Warranty mileage | Up to 450,000 miles | Reduced to 100,000 miles, delayed to MY2030 |
| Deratement (inducement) | Engine slowdown/stop on emissions fault | Replaced with visual/audible warning |
| Useful life provisions | Tighter standards from 2027 | Delayed to 2030 |
| Noncompliance penalties | None specified | Established, allowing payment for NoX exceedance |
Who Is Affected
The proposed rule affects manufacturers of heavy-duty on-road trucks and engines subject to EPA emission standards. The Engine Manufacturers Association, representing these manufacturers, is closely reviewing the NPRM. While the EPA states the amended rule retains 90% of the NoX reductions from the 2023 Final Rule, it projects a “small reduction” in overall NoX reductions compared to earlier estimates. The EPA further estimates that the amended rule would reduce NoX emissions from on-road heavy-duty vehicles by 42% by 2055 relative to a no-rule baseline.
Compliance Obligations and Deadlines
Key compliance deadlines under the proposed changes:
- The broader NoX rule remains effective in 2027.
- The reduced warranty mileage of 100,000 miles applies starting model year 2030.
- Deratement elimination and replacement with warning systems likely align with the main rule’s 2027 effective date, subject to final rule.
- Useful life provisions are delayed to 2030.
Manufacturers should review the full NPRM to determine specific compliance requirements. The 45-day comment period offers an opportunity for industry input.
Penalties for Non-Compliance
The NPRM introduces noncompliance penalties (NCPs), allowing an engine manufacturer to pay an amount for failing to meet NoX standards. The exact penalty amounts are not detailed in the NPRM summary, but the mechanism is intended to provide an alternative to production stoppage. The EPA has not specified whether criminal liability or per-violation fines apply beyond the NCP framework.
Resources and Guidance
The full NPRM (477 pages) is available from the EPA. The Engine Manufacturers Association has issued a preliminary statement and is expected to provide detailed guidance after further review. The public comment period, once the rule is published in the Federal Register, will run for 45 days. Industry associations such as the Truck and Engine Manufacturers Association are also likely to issue compliance resources.
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