Regulations
EPA Proposed Amendments to MY 2027+ Heavy-Duty NOx Program
Source: EPA
Background
On July 8, 2026, EPA Administrator Zeldin signed a proposed rule, “Amendments and Nonconformance Penalties for Model Year 2027 and Later Heavy-Duty Highway Engines and Amendments to Inducement Provisions for SCR-Equipped Diesel Engines.”
The proposal substantially revises several compliance provisions of the Biden-era “2023 Final Rule” while explicitly retaining the MY 2027 emission standards and test cycles.
Comments are due August 29, 2026; virtual public hearings are scheduled for July 29–30, 2026.
Overview & Timing of Proposed Amendments
The proposed amendments apply to medium- and heavy-duty engines, with the DEF-related changes also applicable to non-road engines. These include:
(1) Emission-Related Warranty: Full Rollback to MY 2026 Periods
(2) Regulatory Useful Life: Three-Year Delay to MY 2030
(3) New Nonconformance Penalties (NCPs) for Medium and Heavy HDE Diesel
(4) SCR Inducements: Derates Replaced by Visible/Audible Notifications (we have covered that topic previously)
What’s NOT changing
(1) Emission Limits
(2) Implementation date (except for useful life)
(3) Test Cycles (FTP, Low load cycle) and 2-Bin Off-Cycle In-use testing
(4) Averaging, banking and trading (ABT) architecture
Main proposed amendments of the HD Low NOx rule
Premium Members
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Quick Summary of Changes
Program element | 2023 Final Rule (Biden EPA) | July 2026 Proposal (current EPA) |
NOx / PM / HC / CO standards | NOx 35 mg/hp·hr FTP & SET; 50 mg/hp·hr LLC; Bin 1 idle 10.0 g/hr; Bin 2 58 mg/hp·hr (in-use allowance 50/65/73 for M/H HDE) | UNCHANGED — standards and test cycles not reopened |
Emission-related warranty | From MY 2027: e.g., Heavy HDE 450,000 mi / 10 yr / 22,000 hr (vs. 100,000 mi / 5 yr today) | Rolled back permanently to MY 2026 periods (Heavy HDE 100,000 mi / 5 yr) |
Regulatory useful life | From MY 2027: e.g., Heavy HDE 650,000 mi / 11 yr / 32,000 hr (vs. 435,000 mi / 10 yr) | Longer UL delayed 3 years — applies MY 2030+; MY 2027-2029 keep current UL; comment sought on never extending |
5% production volume allowance (Heavy HDE) | MYs 2027-2029: up to 5% of Heavy HDE volume certified to MY 2026 requirements, offset by NOx credits | Credit-offset requirement removed; 5% cap and other conditions retained |
Nonconformance penalties (NCPs) | None established for MY 2027 standards | New “Phase VIII” NCPs for diesel Medium & Heavy HDE vs. the MY 2027 NOx standard; UL 200 mg/hp·hr; regs migrated to new 40 CFR part 1071 |
SCR/DEF inducements | Codified derate schedule ending at 25 mph (30-164 hr depending on vehicle speed class) | Performance derates replaced with visible and/or audible notifications, required no later than MY 2029; also proposed for LD/MD vehicles and nonroad diesels |
DEF quality monitoring | DEF quality an adjustable parameter; UQS-based precise detection per CISD-13-13 guidance | Detection threshold set at 20% urea; mandatory dosing-compensation algorithms; NOx-sensor detection allowed; freeze-protection codified |
OBD provisions | CARB 2019 OBD incorporated, incl. 0.300 in-use monitor performance ratio (IUMPR) | IUMPR restored to 0.100; comprehensive component monitoring relief for inducement inputs; multi-catalyst monitoring fixes |
ABT credit provisions | FEL caps 65 (MY 2027-30) / 50 (MY 2031+) mg/hp·hr; pre-MY 2022 credits barred; no cross-class credit use | No proposed change — but comment requested on raising FEL cap up to 200, allowing MY 2004-09 credits, and cross-averaging-set use |
What Next?
The proposal lists over 50 questions for which EPA has requested comments. These are due August 29, 2026.
Virtual public hearings are scheduled for July 29–30, 2026.
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