In a dramatic turn of events, California’s ambitious clean truck regulations are facing unprecedented legal and regulatory challenges from the federal government, threatening to dismantle the state’s electric vehicle (EV) truck mandates and reshape its climate strategy. At the heart of the recent controversy is the Clean Truck Partnership (CTP), a 2023 agreement between the California Air Resources Board (CARB) and major truck manufactures, that was designed to ensure industry compliance with California’s Advanced Clean Trucks (ACT) and Heavy-Duty Low NOx Omnibus (Omnibus) regulations, even as federal scrutiny intensified. CARB has leaned heavily on the CTP to continue enforcement of these rules after the federal government revoked its Clean Air Act waivers earlier this year.

However, recent developments suggest the CTP may be unraveling. Four original equipment manufacturers (OEMs)—Daimler Truck, International Motors, PACCAR, and Volvo Group—have now filed a federal lawsuit against CARB, accusing the agency of using the CTP to enforce emissions standards that were invalidated by Congress under the Congressional Review Act (CRA). The lawsuit argues that CARB’s actions violate federal law and place manufacturers in an untenable position. Nevertheless, CARB is standing by its commitments under the CTP and expects manufacturers to do the same. It has recently approved amendments to the ACT (see below) and proposed changes to the Omnibus regulations to offer more compliance flexibility and better align with federal standards.

Adding to the chaos, the Federal Trade Commission (FTC) has declared the CTP agreement to be unenforceable due to antitrust concerns. The FTC found that the CTP’s structure—particularly its “dead hand” provision, which required compliance even if CARB lost legal authority—posed serious risks to market competition and democratic oversight. As a result, the FTC secured written commitments from the four major truck manufacturers that filed the lawsuit and their trade association to withdraw from the agreement and avoid similar arrangements in the future.

This decision has profound implications for California’s clean transportation strategy. With the CTP no longer enforceable, the state’s electric vehicle (EV) sales and purchase mandates are essentially paused. This shift may create space for broader consideration of alternative fuel technologies, including renewable natural gas (RNG) trucks, which offer lower emissions than traditional diesel trucks and are commercially available. In the absence of enforceable EV mandates, policymakers and stakeholders may explore a more diversified approach to achieving near-term emission reductions while continuing to pursue long-term zero-emission goals.

Meanwhile, the U.S. Department of Justice (DOJ) has filed two federal lawsuits against CARB—one targeting its heavy-duty truck emissions standards and another challenging its light-duty vehicle rules. In parallel, the EPA has proposed new rules to curb DEF (Diesel Exhaust Fluid) shutdowns in trucks starting in 2027, a move that could increase diesel truck pollution. Unlike diesel engines, natural gas engines do not rely on SCR (Selective Catalytic Reduction) systems, making them unaffected by this change and potentially more attractive in the evolving regulatory landscape.

The American Trucking Associations (ATA) is also urging the EPA to delay implementation of stricter federal NOx emissions standards until 2031. If granted, this delay would maintain higher NOx limits for several more years, potentially undermining California’s air quality progress if CARB loses its ability to enforce its stricter 50mg NOx standard. However, truck engines such as the Cummins X15N—which meet the 50mg NOx threshold—may continue to play a role in achieving near-term emission reductions, offering a commercially available option that supports cleaner air goals while broader zero-emission technologies continue to scale.

With the CTP’s legal foundation being challenged and federal agencies asserting regulatory control, California’s clean truck future continues to hang in the balance.