Four manufacturers of heavy-duty vehicles and engines have sued the California Air Resources Board and Governor Gavin Newsom in federal court to bar the enforcement of CARB's heavy-duty engine emissions regulations that are preempted under the federal Clean Air Act.

CARB adopted its Advanced Clean Trucks rule, which requires manufacturers to sell an increasing percentage of Zero Emission Vehicles by 2035. Because the CARB requirements are more stringent than federal emission requirements issued by the EPA, in 2023 CARB received a waiver from EPA to implement its regulations. Ten other states have adopted the CARB ACT requirements.

The engine manufacturers and their trade association signed an agreement with CARB called the Clean Truck Partnership in 2023. That agreement gave the manufacturers additional time and certain concessions for compliance with the new California standards. In return, the manufacturers agreed they would follow the CARB standards "irrespective of the outcome of any litigation challenging" them or CARB's authority to implement those standards.

On June 12, 2025, President Trump signed a resolution of disapproval under the Congressional Review Act that rescinded the EPA waiver for the ACT program. The State of California and ten other states have sued, claiming that the recission was unlawful, and that case is ongoing. In the meantime, the congressional recission means that CARB does not have authority to impose its ACT requirements on truck manufacturers. EPA's emissions standards now apply exclusively on a nationwide basis, including in California.

In addition, the U.S. Department of Justice issued a letter to the plaintiff OEMs stating that "Federal law thus prohibits CARB from adopting or attempting to enforce those regulations" governing heavy-duty trucks, and ordering Plaintiff OEMs to cease compliance with the preempted rules.

Nevertheless, CARB has asserted that heavy-duty vehicle and engine manufacturers must continue to follow California's preempted standards to lawfully sell vehicles in the State—including by pointing to the Clean Truck Partnership as a source of its continuing authority. The lawsuit alleges that CARB is attempting to enforce the Clean Truck Partnership as an industry-wide mandate to follow regulations that have been rescinded by congressional act and preempted by federal law.