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The Justice Department and the Department of Transportation sued CARB in March 2026, arguing the Energy Policy and Conservation Act preempts California's fleet CO2 standards and its zero-emission vehicle sales mandate.
In March 2026 the United States sued the California Air Resources Board in federal district court, asking a judge to declare two of California's vehicle regulations unlawful and to permanently enjoin the state from enforcing them. The Department of Justice and the Department of Transportation brought the case together.
The legal theory is preemption, and the statute is not the Clean Air Act. It is the Energy Policy and Conservation Act of 1975.
| Item | Detail |
|---|---|
| Caption | United States v. California Air Resources Board |
| Court | U.S. District Court, Eastern District of California |
| Docket | 26-cv-00847 |
| Filed | March 2026 |
| Plaintiffs | United States, through DOJ and the Department of Transportation |
| Claim | EPCA preemption of state fuel-economy-related regulation |
| Relief sought | Declaratory judgment and a permanent injunction |
| Status | Pending |
Two sets of California regulations are in the crosshairs. The first is the fleetwide carbon dioxide emission standard, codified at Title 13 of the California Code of Regulations, sections 1961.3 and 1961.3.1. The second is the zero-emission vehicle sales mandate at sections 1962.2 and 1962.2.1, which requires an escalating share of each manufacturer's California sales to be zero-emission, climbing toward 100 percent by 2035.
The federal complaint, described by JURIST, argues that both rules are regulations "related to" fuel economy, and that EPCA reserves that field to the federal government.
California's position is that these are emissions rules, that emissions and fuel economy are different subjects even when they correlate, and that the state has regulated vehicle emissions since before the Clean Air Act existed.
EPCA contains an express preemption clause. A state may not adopt or enforce a law or regulation "related to" fuel economy standards for automobiles covered by federal standards. The phrase "related to" is broad on its face, and the fight is over how broad.
The government's argument is arithmetic. Carbon dioxide output from a gasoline engine is a direct function of fuel burned. Regulate one and you have regulated the other. The U.S. Chamber of Commerce filed an amicus brief on August 19, 2026 making that point in one sentence, arguing that "the only way to reduce carbon-dioxide emissions is to reduce the amount of fuel combusted, which increases fuel economy." The Chamber's case page tracks the filing.
California's counterargument is that accepting the arithmetic would preempt any state rule that touches tailpipe emissions, which is not what Congress did when it left California a distinct role in the Clean Air Act.
There is also a threshold question worth flagging. In 2025 the Supreme Court decided Diamond Alternative Energy, LLC v. EPA, No. 24-7, holding that fuel producers had standing to challenge EPA's approval of California standards. That decision was about who may sue, not about whether the standards survive. It matters here because it kept this category of dispute in court rather than ending it on standing.
This case is early. A complaint has been filed, the relief requested is declaratory and injunctive, and amicus briefing was underway by August 2026. No merits ruling has issued.
What that means practically: the next docket entries to watch are California's response, any motion to dismiss, and whether either side moves for a preliminary injunction. A preliminary injunction motion would force a court to say something about likelihood of success within months rather than years, and it would be the first real read on how the judge sees the preemption question.
Expect the district court decision to be a waypoint, not an ending. A ruling either way is going to the Ninth Circuit, and a circuit ruling on a question this large has a plausible path to the Supreme Court.
Manufacturers are the parties with the most immediate exposure. Model year planning runs years ahead of sale, so a rule that might be enjoined in 2027 is already shaping product decisions being made now. That gap between litigation time and industry time is why declaratory relief was requested alongside the injunction.
Also watch the other states. More than a dozen have adopted California's standards under the Clean Air Act's Section 177 mechanism. If EPCA preempts California's rules, the borrowed versions are exposed to the same argument.
Preemption cases generate a heavy paper record, and the substantive arguments arrive in briefs rather than in hearings. The docket is where you see them first.
Pull the current filings here: United States v. California Air Resources Board. To be notified when California answers or when a dispositive motion is briefed, set up case alerts on the case.
Because so much turns on how one judge reads "related to," it is worth pulling that judge's administrative law record before predicting the outcome. That is what the federal judge lookup is for.
Preemption and agency authority are the throughline of the 2026 federal docket. For a tariff case testing the limits of executive power over trade, see Nintendo v. Treasury. For an agency action struck down because the secretary lacked the statutory authority she claimed, read National TPS Alliance v. Noem. And for the constitutional side of the same argument, the Supreme Court's decision on removal protections is covered in Trump v. Slaughter.