Clean Air Act Section 177: Adopting States, Opt-In, and 2025 Waiver

Section 177 of the Clean Air Act is the provision that lets a state step out of the federal vehicle emission scheme and enforce California’s stricter standards instead. The state has to have an approved clean-air plan, its rules must be identical to the California standards the EPA has granted a waiver for, and it has to give manufacturers at least two model years of lead time. Around 17 states and the District of Columbia have used it. A 2025 congressional revocation of several California waivers has thrown enforcement of the newest standards (zero-emission mandates and clean truck rules) into legal limbo while litigation plays out.

What Section 177 Actually Permits

The Clean Air Act generally forbids any state from writing its own emission standards for new motor vehicles.¹ California is the single exception, because it regulated tailpipe pollution before the federal government did; it can apply to the EPA for a waiver to enforce its own rules, and historically the EPA has granted those waivers dozens of times.²

Section 177, codified at 42 U.S.C. § 7507, is the piggyback clause. Once California has a waiver in hand, any other state can adopt those same standards if it meets three conditions.³

It has plan provisions approved under Part D of the Act, the part that covers areas failing to meet national air quality standards. Most states clear this bar.
Its rules are identical to California’s waiver-approved standards. No tweaks, no state-specific add-ons.
Both California and the adopting state finalize the standards at least two full model years before enforcement. A rule completed in late 2024 generally can’t bite until the 2027 model year.

The identicality rule exists to head off what the statute calls a “third vehicle.” If every state could edit California’s rules, a manufacturer might end up building one version of a car for the 49 federal-standard states, a second for California, and a third for a state that adopted California’s rules with its own additions. The statute flatly bars any state action that creates, or has the effect of creating, a vehicle different from one certified in California.³

One point of frequent confusion: a state does not need current nonattainment areas to use Section 177. It needs an approved Part D plan. Clean-air states like Vermont and Maine have adopted California standards on that basis, and reaching attainment later does not strip a state of its Section 177 authority.³

Which States Have Adopted California Standards

As of mid-2025, roughly 17 states plus the District of Columbia had adopted some form of the California program, with the exact count depending on which rule you’re measuring.⁴ About 19 jurisdictions had taken on the Low-Emission Vehicle (LEV) standards for tailpipe pollution. Around 18 had adopted the Zero-Emission Vehicle sales mandate. Eleven had adopted the Advanced Clean Trucks rule for medium- and heavy-duty vehicles.

The adopters cluster in the Northeast and on the West Coast: New York, Massachusetts, New Jersey, Oregon, Washington, Colorado, Virginia, Maryland, Connecticut, Rhode Island, Maine, Vermont, Delaware, Minnesota, Nevada, New Mexico, and Pennsylvania, plus D.C. Arizona and Florida adopted California standards at one point and later repealed them. Together these states make up a large enough share of the new-vehicle market that the California rules effectively shape what automakers design and sell nationally.

How a State Opts In

Adoption is a state regulatory action, not a federal approval. The EPA has confirmed that states do not need EPA sign-off to adopt standards under Section 177.⁵ The work happens inside the state’s own administrative procedure.

Typically the state air or environmental agency drafts regulations that mirror California’s current waiver-approved standards. It publishes the proposal, runs a public comment period of whatever length state law requires, holds hearings where dealers, environmental groups, and residents can testify, and then finalizes the rule after reviewing comments. Any changes have to stay inside the identicality bar.

Because the standards must stay identical to California’s, adoption is not a one-and-done exercise. Every time California updates a regulation and gets a new waiver, a Section 177 state that wants to keep its rules current has to run another rulemaking to pick up the update. States that fall out of sync risk enforcing something that no longer matches California’s rulebook, which puts their legal footing at risk.

What Adoption Means for Drivers and Buyers

Section 177 standards fall on manufacturers, not on individual buyers. The Advanced Clean Cars II regulation, for which the EPA granted a waiver in 2024, requires manufacturers to make sure a rising share of their new light-duty sales are zero-emission or plug-in hybrid: 35% for the 2026 model year, scaling to 100% by model year 2035.⁶ Advanced Clean Trucks applies similar logic to larger vehicles, with 2026 model year targets of 10% for Class 2b-3 trucks, 13% for Class 4-8 trucks, and 10% for Class 7-8 tractors.⁷ A consumer in a Section 177 state is not legally required to buy an electric vehicle. The obligation sits with the manufacturer’s sales mix.

Buyers in Section 177 states do pick up one tangible benefit: better emission warranties. Federal law covers the general emission warranty for only 2 years or 24,000 miles, with major components like catalytic converters and onboard diagnostics covered for 8 years or 80,000 miles.⁸ California’s rules for zero-emission and plug-in hybrid vehicles starting with the 2026 model year warrant propulsion-related parts for 3 years or 50,000 miles, high-priced propulsion components for 7 years or 70,000 miles, and the battery itself for 8 years or 100,000 miles. The battery warranty includes a state-of-health guarantee: it must retain at least 70% of its original capacity through the warranty period, rising to 75% for the 2031 and later model years.⁹ For EV buyers, the capacity guarantee is often the piece that matters most.

The 2025 Waiver Revocation and What’s Still Enforceable

On June 12, 2025, President Trump signed three joint resolutions under the Congressional Review Act that revoked the EPA waivers for California’s Advanced Clean Cars II, Advanced Clean Trucks, and Low NOx regulations. A CRA disapproval blocks the rule from being reissued in the same or substantially similar form.¹⁰

That matters directly for Section 177 states, because Section 177 only lets them adopt California standards “for which a waiver has been granted.” Pulling the waivers pulled the legal floor out from under the adopting states for those specific rules. The ZEV sales mandate, Advanced Clean Trucks, and the Low NOx standard all became unenforceable in every jurisdiction that had adopted them.

California and 10 other states sued in the U.S. District Court for the Northern District of California, arguing that waiver decisions aren’t the kind of agency “rule” the CRA was built to overturn and that the revocation violates the Clean Air Act and the Constitution. As of early 2026 the case is also before the Ninth Circuit, with industry groups intervening on both sides. Until a court resolves it, the newest California-based standards are in limbo and states that had begun enforcing them are waiting to find out whether their rules still have a basis to stand on.

The older LEV standards that have governed conventional tailpipe pollution from gasoline and diesel vehicles for decades were not touched by the CRA resolutions. Section 177 states can still enforce those foundational limits, even while the newer zero-emission requirements sit in legal uncertainty.

1
Office of the Law Revision Counsel. 42 USC 7543 – State Standards
2
U.S. Government Accountability Office. Observations Regarding the Environmental Protection Agencys Submission of Notices of Decision on Clean Air Act Preemption Waivers as Rules Under the Congressional Review Act
3
Office of the Law Revision Counsel. 42 USC 7507 – New Motor Vehicle Emission Standards in Nonattainment Areas
4
U.S. Department of Energy. Adoption of Californias Clean Vehicle Standards by State
5
U.S. Environmental Protection Agency. Vehicle Emissions California Waivers and Authorizations
6
Legal Information Institute. 13 CCR 1962.4 – Zero-Emission Vehicle Requirements for 2026 and Subsequent Model Year Passenger Cars, Light-Duty Trucks, and Medium-Duty Vehicles
7
California Air Resources Board. Advanced Clean Trucks (ACT) Regulation Summary
8
Office of the Law Revision Counsel. 42 USC 7541 – Compliance by Vehicles and Engines in Actual Use
9
Legal Information Institute. Cal Code Regs Tit 13, 1962.8 – Warranty Requirements for Zero-Emission and Batteries in Plug-in Hybrid Electric 2026 and Subsequent Model Year Passenger Cars and Light-Duty Trucks
10
Congressional Research Service. California and the Clean Air Act (CAA) Waiver