EPA proposes shifting Tier 4 vehicle pollution to MY 2029; comments due July 6

EPA has proposed a two-part update to air-pollution rules for certain new cars and trucks. In Part 1, the agency would delay the Tier 4 criteria-pollutant phase-in and keep covered vehicles under Tier 3 requirements longer.

This matters because the rule sets when stricter certification-linked emissions controls would begin for covered vehicle model years—information automakers and suppliers rely on for planning compliance steps like testing, documentation, and production scheduling.

What’s changing in the timeline

Under EPA’s proposed Part 1 rule, the agency would extend Tier 3 through model years (MY) 2027 and 2028. Then, if finalized, the Tier 4 criteria pollutant phase-in would begin in MY 2029—instead of starting earlier under the prior schedule.

Because this is a proposal, MY 2029 is not guaranteed yet. EPA will review public comments before it decides what to finalize.

It’s more than a calendar shift—EPA also adjusts protocols

EPA says Part 1 would also change some test protocols and certification-related program elements so they align with the revised model-year schedule tied to the delayed Tier 4 phase-in.

The practical takeaway for regulated entities is that the timing shift could affect when manufacturers must update or re-run certification testing approaches, reporting, and related compliance processes.

Who is affected

The proposal is aimed at the parties that certify and sell covered light- and medium-duty vehicles under EPA’s onroad emissions program—primarily manufacturers (and other regulated entities connected to certification and compliance).

For shoppers and drivers, the most direct way to think about it is by model year and EPA vehicle coverage categories, not by brand.

Rulemaking status: hearing held; comments due July 6

EPA held a public hearing on June 3–4, 2026. The proposal’s public comment deadline was July 6, 2026 (11:59 p.m. ET).

What to watch next

EPA has described Part 1 as the first step in a two-part process. The next major unknown is Part 2, which EPA says would revisit longer-term Tier 4 program elements and standards for later model years.

Next key questions: what changes EPA makes in response to comments, and how (or whether) Part 2 reshapes the longer-term Tier 4 program beyond the near-term timing shift.

Sources

• EPA: Public Hearing & Public Comments (Tier 4 Part 1)
• Federal Register: Proposed rule PDF (EPA–HQ–OAR–2025–3297)