Since January 2025, there has been a flurry of activity surrounding California’s emissions standards for on-road vehicles. With the California Air Resources Board (CARB) repealing some rules entirely, and actively enforcing others despite pending litigation against them, automakers and fleet owners and operators may naturally be confused about what obligations remain in place.
Advanced Clean Fleets: Running on Fumes, But Still Moving Forward
With an impending June 16, 2026 deadline to comment on recently proposed revisions, the Advanced Clean Fleets (ACF) regulation has been transformed dramatically over the past three years. Adopted in June 2023, the ACF rules originally required truck fleet owners to gradually phase out internal combustion engines and incorporate more zero emission vehicles (ZEVs) into their fleets. The program applied to a broad range of fleets, including drayage truck fleets, state and local government agency fleets, and so-called “high priority” fleets (those with 50 or more trucks, owned by the federal government, or belonging to entities with more than $50 million in gross annual revenues). Moreover, the rules applied to any fleets that traveled into California, even if the trucks were registered elsewhere.
In the face of legal challenges to the ACF rules in both federal and state courts, CARB submitted a request for a waiver of preemption under the federal Clean Air Act (CAA) in November 2023 to bolster its authority for the program. In January 2025, however, CARB withdrew its waiver request, anticipating that the incoming Trump Administration would not be inclined to grant it.
CARB then settled several of the lawsuits challenging the ACF rules by committing to not enforce the provisions of the program related to the high priority and drayage truck fleets. In September 2025, CARB approved amendments formally repealing those requirements, “providing greater certainty to those entities that they do not need to demonstrate compliance.”
Today, CARB is still enforcing the provisions of the ACF rules related to state and local government agency truck fleets, but those provisions have been scaled back and revised. The state legislature passed a law in October 2023 that sought to increase flexibility under the ACF rules for public agency utilities, allowing them to replace certain vehicles at the end of their useful lives with internal combustion engine trucks if necessary to maintain reliable service or respond to emergencies and to use an alternative compliance pathway (the “milestone” pathway for ZEV percentages of the fleet).
Still, significant concerns remain for public agencies and private companies that contract with public agencies to provide fleet services. Proposed revisions this spring—including the recent revisions published June 1, 2026—have made clear that private companies contracting with public agencies to provide fleets would be subject to the requirements of the regulation to transition to ZEV fleets. In other words, private fleet contractors will have to ensure that they enable public agencies to meet their ZEV fleet obligations.
With concurrent changes and challenges to the Advanced Clean Trucks regulation, there could be hurdles to acquiring suitable ZE trucks in the future. And while exemptions have been amended and some of the compliance deadlines have been revised, the ACF as currently proposed still poses significant challenges to government fleets and their private-sector fleet providers. CARB is accepting comments on the recent amendments through June 16, 2026; a hearing on their adoption is not yet scheduled.
Advanced Clean Trucks: Driving Ahead on Shaky Ground
The Advanced Clean Trucks (ACT) program was adopted in June 2020 as a counterpart to the ACF program. While the ACF required fleet owners to purchase a certain percentage of ZEV trucks, the ACT required truck manufacturers to produce and sell a certain percentage of ZEVs. The program sought to ensure a sufficient supply for fleet owners to comply with their corresponding ACF obligations and to reduce NOx and greenhouse gas (GHG) emissions from heavy-duty trucks in the state.
The Heavy-Duty Low NOx Omnibus regulation similarly required manufacturers to meet increasingly stringent NOx emissions standards. In 2023, the Truck and Engine Manufacturers Association and several heavy-duty truck manufacturers entered into the Clean Truck Partnership with CARB, committing to abide by CARB’s truck standards and ZEV sales targets even if those standards were later challenged in court. In exchange, CARB agreed to provide more lead time to meet the standards and to support development and adoption of ZEV infrastructure.
The Biden Administration granted waivers of CAA preemption for both ACT and the Omnibus program, as well as the Advanced Clean Cars II (ACC II) program, in December 2024. In May 2025, however, Congress used the Congressional Review Act (CRA) to rescind the waivers, purporting to entirely void all three. California and a coalition of 10 additional states filed a lawsuit against the federal government challenging the recission, arguing that CAA waivers have never been subject to the CRA. The case is currently pending in the Northern District of California.
With the ACT waiver rescinded, there are two pending challenges to CARB’s enforcement of the Clean Truck Partnership, with manufacturers asserting that they should no longer be bound by the agreement to meet the ZEV sales targets. Without regulatory or transactional obligations to manufacture ZE trucks, and with shrinking incentives to purchase vehicles and install charging infrastructure, options may be reduced and more expensive for those fleets that have to comply with what remains of the ACF.
Meanwhile, California Governor Gavin Newsom issued Executive Order N-27-25 in June 2025, reaffirming California’s commitment to transitioning to ZEVs as embodied in the earlier Executive Order N-79-20, which sets goals of requiring that all sales of new vehicles must be ZEV by 2035 and all medium- and heavy-duty vehicles sold in the state must be ZEV by 2045.
Despite the legal limbo the ACT currently sits in, CARB appears to be reserving its rights to enforce the ACT as indicated by a manufacturers advisory correspondence issued in August 2025. While CARB proposed incorporating certain amendments and flexibilities to the ACT program in 2025, CARB withdrew these proposed amendments on April 29, 2026.
Advanced Clean Cars II: Undergoing a Regulatory Tune-Up
The ACC II rules were adopted in 2022. The program was an update to the original rules adopted in 2012 to address emissions from light-duty vehicles operating in California. The ACC I program required vehicle manufacturers to meet increasingly stringent low-emission vehicle (LEV) standards for GHGs and criteria pollutant emissions as well as ZEV sales goals. ACC II put in place more rigorous benchmarks beginning with model year 2026. In particular, ACC II required that 100% of new light-duty vehicle sales must be ZEVs by model year 2035.
As with the ACT and Omnibus programs, Congress formally rescinded the Biden Administration’s CAA waiver for the ACC II program in May 2025. California’s lawsuit challenging the recission for the ACT and Omnibus program also seeks to reinstate CARB’s authority to enforce the ACC II requirements. In the meantime, however, CARB has readopted the vehicle emissions standards that were in place under the ACC I program and is working to develop a “new phase” of light-duty vehicle emissions standards, called the Drive Forward program, that would apply to criteria pollutants, GHGs, and other toxic emissions.
Why This Matters: Where the Rubber Meets the Road
The road ahead for automakers is uncertain. Manufacturers and fleet owners are understandably unclear about what obligations continue in force today—let alone what obligations they need to be prepared to comply with five or 10 years from now as they plan for future model years. And the confusion extends beyond California. As of April 2026, 10 additional states have adopted California’s ACT program, 11 have adopted the Omnibus rules, and 12 have adopted ACC II. Together, these states represent 28.6% of new heavy-duty vehicle registrations and 40.7% of new light-duty vehicle registrations. Under the current legal uncertainty, however, some of these states are backing off their decisions to follow California. For example, lawmakers and regulators in Washington State, Vermont, and Oregon have each taken steps to stop enforcement of ACT or ACC II obligations. These decisions provide some regulatory relief for the auto industry, but they may add to the confusion as well.
Ongoing litigation further complicates the calculus. California and its coalition of states are actively pursuing their claims against the federal government over their authority to enforce the ACT, ACC II, and Omnibus programs. In addition, four truck manufacturers that had previously signed on to the Clean Truck Partnership have filed suit in the Eastern District of California, asking the court to enjoin CARB from enforcing their commitments under the agreement given Congress’s withdrawal of the CAA waiver that supported their obligations. The district court partially granted the truck manufacturers’ motion for a preliminary injunction against enforcement of the Clean Truck Partnership agreement in October 2025. Motions for summary judgment were filed by the truck manufacturer coalition, CARB, and the EPA; a hearing on the motions is set for August 7, 2026.
The Road Ahead
While California litigates the future of the ACT and ACC II programs, CARB is simultaneously retooling its approach to regulating both light- and heavy-duty vehicle emissions. Beginning in December 2025, CARB began holding public workshops on a Drive Forward Fleet initiative, and additional listening sessions have been scheduled for the light-duty Drive Forward program. These efforts are in their earliest stages, and the specific obligations for manufacturers and fleet owners will likely not be proposed for several years. Meanwhile, government fleets—including private contractors of those fleets—are still likely to face compliance obligations under the revamped ACF.
If you have any questions, or would like additional information, please contact one of the attorneys on our Environment, Land Use & Natural Resources team.
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