What: On May 18, 2026, the U.S. Environmental Protection Agency (EPA) announced a proposal to delay Tier 4 standards for model year (MY) 2027 and later light and medium-duty vehicles.
Why: Under the Clean Air Act (CAA), the EPA is required to set vehicle emissions standards for pollutants contributing to ozone and particulate matter. These standards must consider feasibility, cost, and lead time.
The current Tier 4 standards for light and medium-duty vehicles were set in 2024 and are scheduled to phase in from MY 2027 through MY 2032. The EPA’s proposal would delay the Tier 4 standards until MY 2029. During this time, all light and medium-duty vehicles would continue to comply with the existing Tier 3 standards, which were finalized in 2014.
The EPA states that the Tier 4 standards were based on flawed assumptions regarding the number of electric vehicles that would be available. In a forthcoming rulemaking, the EPA plans to comprehensively reconsider the Tier 4 program. The reconsideration may include changes to the standards, implementation dates, phase-in schedules, and test procedures.
The EPA will hold a virtual public hearing on June 3-4, 2026. To request information on registering for the public hearing, send an email to EPA-LD-MobileSource-Hearings@epa.gov. Registration must be completed by May 27, 2026.
Action: Comments are due July 6, 2026.
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