The Clean Air Act Section 209 waiver is the federal mechanism that allows California, and only California, to enforce its own vehicle emission standards after receiving approval from the U.S. Environmental Protection Agency. Every other state is preempted from setting its own tailpipe rules for new motor vehicles. California qualifies for the exception because it began regulating vehicle pollution in 1966, before the federal program existed, and Congress preserved that head start when it wrote the modern Clean Air Act.1California Air Resources Board. History The waiver has been granted routinely for most of its history, revoked once during the first Trump administration, reinstated under President Biden, and — for several of California’s most ambitious programs — nullified by Congress in 2025.
Why Only California Can Apply
Section 209(a) of the Clean Air Act bars any state or local government from adopting or enforcing its own emission standards for new motor vehicles. The reason is practical: without a single national rule, automakers would face fifty different standards, and no state can require its own emission certification as a condition of a new vehicle’s first sale.2Office of the Law Revision Counsel. 42 USC 7543 – State Standards
Section 209(b) opens a door for any state that had adopted vehicle emission standards before March 30, 1966. California is the only state that meets that cutoff. To seek a waiver, the California Air Resources Board must first determine that its proposed standards, taken as a whole, are at least as protective of public health as the corresponding federal rules. That “in the aggregate” comparison is important: CARB doesn’t have to show that every individual standard is tougher than the federal version. The whole package just has to be at least as strong overall.2Office of the Law Revision Counsel. 42 USC 7543 – State Standards
The Three Grounds for Denial
The statute favors California. The EPA Administrator is required to grant the waiver unless one of three specific findings supports denial, and the burden sits with the federal government to justify a “no.”
- The Administrator finds that California’s protectiveness determination was arbitrary and capricious, meaning the state lacked a rational basis for concluding its standards are at least as protective as federal standards.2Office of the Law Revision Counsel. 42 USC 7543 – State Standards
- The Administrator finds that California no longer needs its own standards to meet compelling and extraordinary conditions. This turns on the unique geography and climate that concentrate pollution in the state’s air basins.3Federal Register. California State Motor Vehicle Pollution Control Standards – Notice of Decision Granting a Waiver of Clean Air Act Preemption
- The Administrator finds that California’s standards are inconsistent with Section 202(a), which requires emission rules to allow adequate lead time and give appropriate consideration to the cost of compliance. This is the technological feasibility test.4Office of the Law Revision Counsel. 42 USC 7521 – Emission Standards for New Motor Vehicles
The “compelling and extraordinary conditions” prong has never successfully blocked a waiver in practice. Most serious disputes land on technological feasibility and cost, because that’s where the EPA has to decide whether automakers can actually build vehicles meeting California’s targets at a reasonable price.
How a Waiver Request Moves Through the EPA
Before anything reaches Washington, CARB builds an administrative record justifying the proposed regulations. That record includes a protectiveness determination comparing California’s rules to the full set of federal standards, quantifying reductions in pollutants such as nitrogen oxides, particulate matter, carbon monoxide, and greenhouse gases. CARB also prepares an economic analysis estimating how the standards will affect new vehicle prices and whether the costs are proportionate to the health and environmental gains. And the application has to show that the auto industry can realistically hit the new targets using existing or near-future technology.
Once the application arrives, the EPA publishes a notice in the Federal Register and opens a public process. The agency holds at least one public hearing where automakers, environmental groups, states, and other parties can present testimony, and a written comment period stays open afterward. EPA staff then evaluate the record against the three statutory criteria, and the Administrator’s final decision — grant or denial — is published in the Federal Register.5U.S. Environmental Protection Agency. Vehicle Emissions California Waivers and Authorizations
Revocation, Reinstatement, and the 2025 Congressional Action
The waiver process was largely routine for decades. That changed during the first Trump administration. In September 2019, the EPA withdrew California’s existing waiver for greenhouse gas emission standards and zero-emission vehicle requirements through the SAFE Vehicles Rule Part One, arguing that California no longer needed its own standards for those pollutants and that the standards were preempted by federal fuel economy law.6Federal Register. The Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule Part One It was the first time the federal government had ever revoked a previously granted Section 209 waiver.
The Biden EPA reversed course in March 2022, formally rescinding the SAFE rule’s waiver withdrawal and reinstating California’s authority over greenhouse gas and zero-emission vehicle standards.7U.S. Environmental Protection Agency. EPA Restores California’s Authority8California Air Resources Board. 1962.4 ZEV Standards 2026+9Federal Register. Advanced Clean Cars II Waiver
The most consequential shift came in 2025, and it came from Congress, not the EPA. Both chambers passed joint resolutions under the Congressional Review Act to nullify the EPA’s waivers for three California programs: Advanced Clean Cars II, Advanced Clean Trucks, and the Omnibus Low NOX regulations. President Trump signed the resolutions, declaring that all three programs “are fully and expressly preempted by the Clean Air Act and cannot be implemented.”10The White House. Statement by the President
The Congressional Review Act carries a sharp aftershock. Once a rule is disapproved, the issuing agency cannot adopt any future rule that is “substantially the same” without new legislation. The President’s signing statement read that prohibition broadly, asserting that the EPA is now blocked from approving future waivers allowing California to regulate greenhouse gas emissions from internal combustion engines or to impose what he characterized as a nationwide electric vehicle sales mandate.10The White House. Statement by the President Whether courts adopt that sweeping reading is an open question.
The Section 209(b) mechanism itself still exists. California retains waivers covering earlier programs for conventional tailpipe pollutants, and the statutory framework remains available for future applications. But any new waiver request touching greenhouse gases or zero-emission vehicle mandates now runs into the CRA’s “substantially the same” bar on top of the usual EPA review.
How Other States Plug In Through Section 177
Only California can apply for a Section 209 waiver. Section 177 lets other states adopt California’s approved standards without seeking their own waiver — they ride on California’s.11Office of the Law Revision Counsel. 42 USC 7507 – New Motor Vehicle Emission Standards in Nonattainment Areas More than a dozen states have used this authority for various California programs, which is what gives California outsized reach across the national auto market.
Section 177 comes with two firm limits. Any adopting state must use standards identical to California’s in every technical respect. No modifications. And the adopting state must finalize its rules at least two years before the model year they apply to. The statute also prohibits any state from creating a “third vehicle” configuration different from both the federal-certified and California-certified versions, so automakers only need to build two versions for the U.S. market.11Office of the Law Revision Counsel. 42 USC 7507 – New Motor Vehicle Emission Standards in Nonattainment Areas
The 2025 CRA revocations have unsettled Section 177 states. If California’s underlying waiver is nullified, the legal basis for other states enforcing identical standards falls with it. States that had adopted Advanced Clean Cars II or Advanced Clean Trucks provisions now face questions about whether their own regulations remain enforceable and whether automakers or dealers will challenge them.
Nonroad Engines Follow a Similar Track
California’s authority reaches beyond passenger cars. Section 209(e) covers nonroad engines and vehicles: construction equipment, forklifts, airport ground support, and similar machinery. For this category, California requests an “authorization” rather than a “waiver,” and the EPA applies criteria that mirror the on-road test — grant unless the state’s determination was arbitrary and capricious, unless compelling and extraordinary conditions are absent, or unless the standards are inconsistent with the Act.12eCFR. 40 CFR Part 1074 – Preemption of State Standards and Procedures for Waiver of Federal Preemption for Nonroad Engines and Nonroad Vehicles
The scope of preemption is narrower for nonroad equipment. States are completely barred, with or without authorization, from regulating emissions from new farm and construction engines under 175 horsepower and from new locomotive engines. Other states can adopt California’s authorized nonroad standards under the same identical-and-two-years-ahead rules that apply to on-road vehicles.13eCFR. 40 CFR Part 1074
Where Waiver Disputes Get Decided
Challenges to EPA waiver decisions bypass the trial courts. Under Section 307(b)(1) of the Clean Air Act, petitions for review of nationally applicable EPA actions go directly to the U.S. Court of Appeals for the District of Columbia Circuit.14Office of the Law Revision Counsel. 42 USC 7607 – Administrative Proceedings and Judicial Review Because waiver decisions affect the entire national auto market, the D.C. Circuit has been the primary battleground. When Ohio led seventeen states in challenging the Biden EPA’s 2022 reinstatement, the D.C. Circuit found the states had standing but ruled for the EPA on the merits, and the Supreme Court declined to hear the case. States, automakers, fuel producers, and environmental organizations have all appeared in waiver litigation, and the range of parties with standing to challenge these decisions is broad.