The Trump administration is suing the California Air Resources Board (CARB) to block enforcement of the state’s vehicle emissions rules. The U.S. Department of Justice (DOJ) argues that the state’s fleetwide CO2 emissions regulation and zero-emission vehicle (ZEV) sales mandate conflict with federal law governing fuel economy standards.
The complaint, filed on behalf of the National Highway Traffic Safety Administration (NHTSA), argues that California’s CO2 and ZEV rules function as de facto fuel-economy standards for automakers. Under the federal Energy Policy and Conservation Act (EPCA), which authorizes nationwide Corporate Average Fuel Economy (CAFE) standards, only the federal government can regulate vehicle fuel economy.
In June 2025, Congress revoked California’s vehicle emissions waiver that previously allowed the state to enforce its ZEV mandate. In response, CARB issued emergency regulations in October 2025 to continue enforcing the Advanced Clean Cars II rule, which requires the phase-out of the sale of new gasoline-fueled or diesel-fueled passenger cars and trucks by 2035, and the Heavy-Duty Omnibus rule regulating heavy-duty engines.
The federal government argues that California’s regulations are preempted by EPCA because they are directly related to fuel economy. According to the complaint, “reducing or eliminating tailpipe CO2 emissions from internal combustion automobiles effectively increases fuel economy.”
The complaint states that CARB’s CO2 standards and ZEV mandates “undermine and conflict with NHTSA’s congressionally assigned role in establishing nationwide, uniform vehicle fuel economy standards.” The regulations also “create a patchwork of inconsistent regulation for vehicle and engine manufacturers in an area where Congress imposed a uniform, national approach.”
According to the filing, the regulations “harm the interests of the United States in ensuring American consumers’ access to and choice of reliable, affordable motor vehicle.”
The government also argues that California’s waivers under the Clean Air Act exempt the state from only the Clean Air Act‘s preemption provisions. “Such a waiver has no effect on EPCA,” the complaint states.
CARB has long maintained that its vehicle rules fall within the state’s authority to regulate emissions in order to address air pollution and climate change. State officials have historically argued that the regulations establish emissions standards, not fuel-economy requirements. (For more on California’s auto emissions waiver, see California’s Energy Transition: Part Four—Auto Emissions.)
CARB is currently developing new regulatory proposals in response to Governor Gavin Newsom’s Executive Order N-27-25, issued after Congress revoked California’s emissions waiver. The complaint criticizes those efforts and states that CARB “will continue these unlawful actions unless the Court intervenes.”