The Clean Air Act Section 209 waiver is the federal mechanism that lets California set and enforce vehicle emission standards stricter than the national rules. Section 209(a) preempts every state from regulating new motor vehicle emissions, but Section 209(b) carves out an exception for any state that had its own standards in place before March 30, 1966. Only California qualifies.1Office of the Law Revision Counsel. 42 USC 7543 – State Standards California applies to the EPA, the EPA grants or denies, and other states can then adopt California’s rules in place of the federal ones. In 2025, Congress used the Congressional Review Act to nullify EPA waivers for three major California programs, significantly narrowing what the state can currently enforce.
Why Only California
Without preemption, automakers would face up to fifty different rule sets for the same vehicle. Section 209(a) prevents that by barring states from adopting or enforcing their own emission standards for new motor vehicles, including any state certification or inspection as a condition of first sale or registration.1Office of the Law Revision Counsel. 42 USC 7543 – State Standards
The exception in Section 209(b) is historical. California began regulating tailpipe pollution in 1966, before the federal program existed, so Congress let it keep going on the condition that the EPA sign off.2California Air Resources Board. History Before requesting a waiver, the California Air Resources Board must find that its proposed standards, taken as a whole, are at least as protective of public health as the federal rules. This “in the aggregate” test means an individual California standard can be weaker than its federal counterpart as long as the overall package is at least as strong.
The Three Grounds for Denial
The statute puts the burden on the EPA to justify saying no. The Administrator must grant a waiver unless one of three findings applies:
- California’s protectiveness determination was arbitrary and capricious.1Office of the Law Revision Counsel. 42 USC 7543 – State Standards
- California does not need its own standards to meet compelling and extraordinary conditions, meaning the unique geographic and climatic factors (coastal basins that trap pollution, temperature inversions, strong sunlight that drives smog formation) that have historically justified separate rules.3Federal Register. California State Motor Vehicle Pollution Control Standards – Notice of Decision Granting a Waiver of Clean Air Act Preemption for Californias 2009 and Subsequent Model Year Greenhouse Gas Emission Standards for New Motor Vehicles
- The standards are inconsistent with Section 202(a), which requires that emission standards allow adequate lead time and give appropriate consideration to the cost of compliance. A waiver can be denied under this prong if the technology to meet California’s rules doesn’t exist and can’t be developed in a reasonable time, or if compliance costs are excessive relative to the benefits.4Office of the Law Revision Counsel. 42 USC 7521 – Emission Standards for New Motor Vehicles or New Motor Vehicle Engines
In practice, the compelling and extraordinary conditions prong has never successfully blocked a waiver. California’s geography has not changed, and its air quality challenges, though much improved, remain among the worst in the country. Most serious disputes land on the technological feasibility prong.
How the Application Moves Through EPA
CARB builds the record. State engineers quantify how much each pollutant would be reduced using laboratory testing and atmospheric modeling, and CARB prepares an economic analysis of what the standards add to new vehicle prices relative to the health and environmental gains. The application must also show that automakers can realistically meet the targets with existing or near-future technology.
Once the application reaches Washington, the EPA publishes notice in the Federal Register and opens the process to public participation, including at least one public hearing where automakers, environmental groups, and others testify. A written comment period follows. EPA staff then evaluate the record against the three statutory criteria, and the Administrator’s grant or denial is published in the Federal Register.5U.S. Environmental Protection Agency. Vehicle Emissions California Waivers and Authorizations
How Other States Opt In
Section 177 lets any state adopt California’s approved standards instead of the federal program. The adopting state does not need its own waiver; it rides on California’s.6Office of the Law Revision Counsel. 42 USC 7507 – New Motor Vehicle Emission Standards in Nonattainment Areas More than a dozen states have used Section 177 for various California programs, which is why California effectively shapes the national auto market.
Two firm limits apply. The adopting state’s rules must be identical to California’s in every technical respect, with no state-specific modifications. And the rules must be finalized at least two years before the model year they apply to. The statute also prohibits creating a “third vehicle” that differs from both the federal and California configurations, so automakers only have to build two versions for the U.S. market.6Office of the Law Revision Counsel. 42 USC 7507 – New Motor Vehicle Emission Standards in Nonattainment Areas
Nonroad Engines Use a Separate Track
Section 209 is not just a passenger-car rule. Section 209(e) covers nonroad engines and vehicles such as construction equipment, forklifts, and airport ground support vehicles. For this category, California requests an “authorization” rather than a “waiver,” evaluated against essentially the same three criteria.7eCFR. 40 CFR Part 1074 – Preemption of State Standards and Procedures for Waiver of Federal Preemption for Nonroad Engines and Nonroad Vehicles
Two nonroad categories are entirely off-limits to state regulation regardless of any authorization: new farm and construction engines under 175 horsepower, and new locomotive engines. For small spark-ignition engines under 50 horsepower, like those in lawnmowers and leaf blowers, the EPA must weigh safety concerns such as fire or burn risk when evaluating California’s request. States other than California generally cannot regulate those small engines unless they had standards in place before September 2003.7eCFR. 40 CFR Part 1074 – Preemption of State Standards and Procedures for Waiver of Federal Preemption for Nonroad Engines and Nonroad Vehicles
Where the Waiver Stands After 2025
For most of the program’s history, waiver grants were routine. That changed in September 2019, when the first Trump administration’s EPA withdrew California’s existing waiver for greenhouse gas and zero-emission vehicle standards through the SAFE Vehicles Rule, Part One. It was the first time a previously granted Section 209 waiver had been revoked.8Federal Register. The Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule Part One – One National Program The EPA has taken the position that it can reconsider a previously granted waiver when there is factual error or significantly changed circumstances, even though the statute does not spell out that authority.
In March 2022, the Biden EPA rescinded the SAFE withdrawal and reinstated California’s authority to enforce its greenhouse gas and zero-emission vehicle standards.9U.S. Environmental Protection Agency. EPA Restores Californias Authority to Enforce Greenhouse Gas Emission Standards for Cars and Light Trucks In December 2024, the EPA granted California’s Advanced Clean Cars II waiver, which required rising percentages of zero-emission vehicle sales starting with model year 2026 and reaching 100 percent by model year 2035.10Federal Register. California State Motor Vehicle and Engine Pollution Control Standards – Advanced Clean Cars II Waiver
The biggest change came from Congress. In 2025, both chambers passed Congressional Review Act joint resolutions nullifying the EPA’s waivers for three programs: Advanced Clean Cars II, Advanced Clean Trucks, and the Omnibus Low NOX regulations. President Trump signed the resolutions and declared that the three programs are “fully and expressly preempted by the Clean Air Act and cannot be implemented.”11The White House. Statement by the President
The CRA has a sharp aftershock. Once a rule is disapproved, the issuing agency cannot adopt a future rule that is “substantially the same” without new legislation. The signing statement read that bar broadly, asserting that the EPA cannot now approve future waivers allowing California to regulate greenhouse gas emissions from internal combustion engines or to impose what amounts to a nationwide electric vehicle sales mandate.11The White House. Statement by the President Whether courts will accept that sweeping reading is unresolved.
The waiver mechanism itself still exists. California retains waivers for earlier programs covering conventional tailpipe pollutants, and the statutory framework remains available for future applications. But Section 177 states that had adopted the nullified programs now face real uncertainty about whether their identical rules remain enforceable, because when California’s underlying waiver disappears, the legal basis for the adopting state’s rule disappears with it.
Where Challenges Are Heard
Challenges to EPA waiver decisions go directly to a federal appeals court, not to a trial court. Under Section 307(b)(1) of the Clean Air Act, petitions for review of nationally applicable EPA actions must be filed in the U.S. Court of Appeals for the District of Columbia Circuit.12Office 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, and standing has been read broadly enough that states, automakers, fuel producers, and environmental organizations have all participated.