Key Points:

• The change in Presidential Administration has led to uncertainty regarding several California vehicle emissions regulations requiring federal waivers under the Clean Air Act, as the Trump Administration is attempting to reverse prior approvals of some regulations, and California has voluntarily withdrawn requests for approval of others.
• The Supreme Court recently declined to hear cases challenging the constitutionality of California’s Clean Air Act waiver, leaving in place a DC Circuit decision upholding the constitutionality of the waiver.
• The Trump administration has asked Congress to use its powers under the Congressional Review Act to reverse the Biden administration’s approval of California’s Advanced Clean Trucks rule, Advanced Clean Cars II program, and Heavy-Duty Omnibus regulation, though the legal basis for reversal is questionable. For now, these regulations remain in effect in California.
• The Trump administration has not yet taken any actions regarding the Clean Air Act waiver granted for a portion of California’s Commercial Harbor Craft rule, leaving the rule in effect in California.
• California has voluntarily withdrawn its Clean Air Act waiver requests for the Advanced Clean Fleet rule, In-Use Locomotive standards, and a portion of the Commercial Harbor Craft rule. These regulations are not in effect, except for the provisions of the Advanced Clean Fleet rule requiring an increasing proportion of new vehicles in state and local government fleets to be zero-emission vehicles.

Vehicle manufacturers and operators in California are no stranger to complex regulations, with the California Air Resources Board’s (CARB) history of pioneering mobile emission regulations. However, a series of developments over the last four months have made parsing California’s vehicle emissions regulations more challenging than usual, with many proposed and active vehicle regulations thrown into question by the change in presidential administration.

While the Clean Air Act endows the Environmental Protection Agency (EPA) with the sole authority to set national emission standards for vehicles and vehicle engines, Section 209(b) of the Act allows the EPA to authorize a waiver of this federal preemption for California regulations that it determines are at least as protective of public health and welfare as the otherwise applicable Federal standards.¹ Under Section 177 of the Act, other states are allowed to adopt any California regulation authorized by a waiver—essentially allowing California to set alternative nationwide emissions regulations that are stricter than the EPA’s. California has used this waiver provision extensively, with over 100 waivers approved. Thirteen states and the District of Columbia have adopted’ at least some of California’s vehicle emissions standards.

From 2020 to 2024, “CARB” crafted a series of vehicle emissions standards governing passenger vehicles, various medium- and heavy-duty trucks, water craft, and trains, and subsequently submitted these emissions regulations to the EPA for authorization under the Clean Air Act’s waiver provision. However, the prospect of an incoming Trump administration presented a potential obstacle to CARB’s regulatory agenda, prompting the Biden EPA to issue a flurry of waiver approvals prior to January 20, 2025, and led CARB to withdraw other pending waiver requests before the Trump administration took over. Now, the Trump administration has asked Congress to reverse several of the waiver approvals granted by the Biden EPA, adding further uncertainty to the mix.

The upshot is a confusing regulatory landscape for state agencies, local jurisdictions, and regulated businesses alike. Below, we provide a summary of relevant developments over the past few months, the status of several key CARB emissions regulations, and what interested parties can expect going forward.

The Supreme Court Recently Declined to Take Up a Challenge to the Constitutionality of California’s Waivers

In December of 2024, the Supreme Court denied two petitions regarding related cases from the DC Circuit challenging the constitutionality of the EPA’s grant of a waiver allowing California to enact its Advanced Clean Cars I regulatory program, which set emissions standards for passenger vehicles sold through 2025. While a waiver for Advanced Clean Cars I was initially approved in January 2013 by the Obama EPA, the Trump administration effectuated a withdrawal of the waiver as part of its 2019 “One National Program” rule on Federal Preemption of State Fuel Economy Standards. However, the Biden EPA reinstated the waiver in 2022, prompting two lawsuits:

1. Diamond Alternative Energy LLC et al. v. U.S. Environmental Protection Agency (Docket Number 24-7), in which a coalition of oil and gas companies argued that the Biden EPA’s issuance of the waiver exceeded the authority EPA is granted under Section 209(b); and
2. Ohio et al. v. U.S. Environmental Protection Agency (Docket Number 24-13), in which a coalition of states argued that a California-specific waiver violated constitutional equal sovereignty principles.

The Supreme Court denied the Diamond and Ohio petitions to review the merits of the challenges to the waiver program (though it granted review on a narrow question of oil and gas companies’ standing to bring suit in the Diamond case). With the Supreme Court declining to hear the cases, the 2024 DC Circuit’s holding upholding the constitutionality of the waiver program remains in place.

The Trump Administration is Attempting to Reverse the Biden EPA’s Authorization of Key CARB Regulations on Truck, Bus Emissions, and Passenger Vehicle Emissions, Likely Prompting Further Litigation

The Trump Administration has asked Congress to use the Congressional Review Act (CRA) to reverse several waivers issued by the Biden EPA authorizing CARB regulations addressing on-road vehicle emissions.

In 2023, the Biden EPA granted a waiver for CARB’s Advanced Clean Trucks rule, which places regulations on medium- and heavy-duty truck manufacturers who sell trucks within the state. Under the Advanced Clean Trucks rule, manufacturers must sell zero-emissions vehicles or near-zero-emissions vehicles (such as plug-in electric hybrids) as an increasing percentage of their annual sales within the State from 2024 to 2035, or alternatively, must purchase credits from other manufacturers to offset the amount of non-zero emission vehicles sold. Ten other states have opted into at least a portion of the Advanced Clean Trucks rule.

In late December 2024 and early January 2025, the Biden EPA granted waivers for two additional high-profile CARB regulations governing on-road vehicles:

• The Advanced Clean Cars II program, which requires an increasing proportion of new vehicles sold within California to be zero-emission until 2035, when all new vehicles must be zero-emission. Eleven states and the District of Columbia have opted into at least a portion of Advanced Clean Cars II regulations.
• The Heavy-Duty Omnibus regulation, which imposes stringent NO_x emissions regulations for heavy-duty trucks and buses sold within the State.

As of now, all of the above described CARB regulations are in effect. However, the Trump EPA and Congressional Republicans have signaled that they will take steps to reverse the waivers previously granted for the Advanced Clean Trucks rule, Advanced Clean Cars II program, and Heavy-Duty Omnibus regulation, using the CRA.² It is not clear that this reversal attempt is on strong legal footing—a recent Congressional Review Services Report, interpreting a 2023 Government Accountability Office opinion, concluded that the CRA does not authorize review and reversal of waivers because waiver approvals are not “rules” subject to the reach of the CRA.³ If Congress proceeds with reversal of these waivers under the CRA, such action would almost certainly be met with legal opposition, at the very least from California, where Governor Gavin Newsom recently requested $25 Million from the legislature to fund potential legal disputes with the Federal Government.

The Trump Administration has Yet to Take Any Action Regarding the Biden EPA’s Waiver for the Commercial Harbor Craft Rule

In January 2025, the Biden EPA also granted a waiver for a portion of CARB’s Commercial Harbor Craft rule. The waiver allows CARB to impose stringent emissions regulations for certain boats and other watercrafts, including Zero-Emission and Advanced Technology (or ZEAT) requirements for new and in-use ferries and new excursion vessels.

To date, the Trump Administration has not taken any actions to reverse the Biden EPA’s waiver for CARB’s Commercial Harbor Craft rule, and CARB’s regulations remain in place.

CARB Withdrew Three Waiver Requests Related to Truck Fleets, Locomotives, and Portions of the Commercial Harbor Craft Rules Prior to President Trump’s Inauguration

On January 13, 2025, CARB elected to withdraw three waiver requests for emissions regulations that EPA had not yet approved, citing the impending uncertainty presented by the incoming Trump Administration. With the exception of state and local government fleet requirements discussed below, CARB cannot enforce these regulations.

The most prominent withdrawal was CARB’s Advanced Clean Fleet rule, a separate regulation related to the currently-active Advanced Clean Trucks rule targeted for reversal by the Trump administration. Unlike the Advanced Clean Truck rule, which mandates the types of trucks a manufacturer is permitted to sell in California, the Advanced Clean Fleet rule would have regulated the mix of truck types in that fleet operations are permitted to purchase and maintain. For private fleets, drayage trucks fleets at ports or railyards would have been required to be composed entirely of zero-emission trucks (i.e., battery electric, long-range plug-in electric hybrids, and hydrogen fuel cell MDHD trucks) by 2035. Other private fleets would have to meet zero-emission vehicle percentages for different types of vehicles until 2042, when 100 percent of all medium- and heavy-duty internal combustion engine fleets in California would be required to be entirely composed of zero-emission trucks.

Though withdrawal of the waiver request means the majority of the Advanced Clean Fleet rule will not take effect, CARB has announced that it will continue to enforce portions of the rule that apply to state and local government fleets, which it contends do not need a federal waiver. State and government fleets are required to ensure that 50% of their annual vehicle purchases in 2025 and 2026 are of zero-emission vehicles, increasing to 100% starting in 2027. The rule includes exemptions for fleets of 10 or fewer vehicles, fleets located in low population counties, and for certain vehicle types. CARB also withdrew waiver requests to authorize:

1. In-Use Locomotive standards, under which only locomotives less than 23 years old would have been allowed in California beginning in 2030, unless they were zero emissions.
2. Portions of the Commercial Harbor Craft rule that were not previously approved by the EPA, and would have required in-use, short-run ferries to comply with zero emission requirements.

It is unknown whether CARB will try to re-introduce these regulations. Given ’its rationale for withdrawal, it is unlikely that CARB will re-submit waiver requests for these vehicle regulations over the next four years of the Trump administration. In 2019, when faced with an administration unlikely to approve waivers for the passenger vehicle industry, California reached voluntary agreements with Ford, Honda, VW, and BMW for emissions standards more stringent than the federal standards, but less stringent than those initially proposed by CARB. California may attempt a similar maneuver here, seeking voluntary agreements with trucking companies, railroad operators, and other industries while the prospect of Clean Air Act waivers remains unlikely.

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¹ EPA maintains the discretion to deny a waiver if it makes certain determinations regarding the necessity or appropriateness of the waiver request. In practice, the EPA has previously denied only one of California’s more than 100 waiver requests, and that denial was swiftly reconsidered and reversed by the Obama EPA in 2009.

² See “Trump EPA to Transmit California Waivers to Congress in Accordance with Statutory Reporting Requirements,” EPA Press Office, February 14, 2025. Available at:https://www.epa.gov/newsreleases/trump-epa-transmit-california-waivers-congress-accordance-statutory-reporting.

³ “California and the Clean Air Act (CAA) Waiver: Frequently Asked Questions,” Congressional Research Service, August 30, 2024. Available at:https://crsreports.congress.gov/product/pdf/R/R48168.