California Attorney General (AG) Rob Bonta, the California Air Resources Board (CARB), and a coalition of 19 states and four cities have urged the U.S. Environmental Protection Agency (EPA) to move forward with its Tier 4 vehicle emissions standards for light- and medium-duty vehicles. The coalition’s comments respond to EPA’s proposal to delay implementation of Tier 4 from model year 2027 to model year 2029 while the agency reconsiders the broader program. During the proposed delay, manufacturers would continue complying with the current Tier 3 emissions standards.

Adopted in 2024, Tier 4 is EPA’s next-generation criteria pollutant program for passenger cars, SUVs, pickup trucks, vans, and medium-duty vehicles. The standards tighten limits on conventional pollutants such as nitrogen oxides (NOx), particulate matter (PM), hydrocarbons, and carbon monoxide, with the goal of improving air quality and reducing public-health impacts from vehicle emissions.

In their comment letter, AG Bonta, CARB, and allied stakeholders argue that delaying the rule would postpone emissions reductions, harm public health, slow air-quality improvements, and undermine regulatory certainty for states and manufacturers that have already been planning around the 2027 schedule. EPA, however, has taken the position that manufacturers and suppliers need additional time to adapt to changing market conditions, technological and feasibility challenges, and evolving regulatory requirements. The federal agency argues that retaining Tier 3 standards through model years 2027 and 2028 would maintain significant emissions protections while EPA conducts a separate rulemaking to reconsider the longer-term Tier 4 program.

The proposed delay comes as EPA is also reviewing other major vehicle emissions regulations. Most notable is a similar yet separate rule finalized by the Biden Administration that establishes a significantly stricter NOx emissions standards for heavy-duty trucks and buses beginning with model year 2027. EPA’s Heavy-Duty NOx Rule is intended to reduce emissions from one of the nation’s largest sources of ozone-forming pollution and improve air quality in communities located near major freight corridors. While EPA has not proposed eliminating those standards, stakeholders are closely watching for potential revisions as the agency reevaluates several mobile-source regulations.