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What Standards Apply? CARB Moves to Clarify Emissions Rules Amid Legal Limbo

California is issuing emergency rules reinstating prior emissions standards to ensure vehicles can still be sold in California amid unprecedented legal uncertainty.

Deborah Lockridge
Deborah LockridgeEditor and Associate Publisher
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September 16, 2025
Illustration of Los Angeles smog, truck exhaust smoke, and CARB logo

Are any California truck emissions rules still in effect? CARB proposes emergency regulations to clarify.

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4 min to read


The California Air Resources Board has proposed emergency vehicle emissions regulations to clarify the situation as it challenges actions taken earlier this year to revoke previously issued Clean Air Act waivers for California’s Heavy Duty Low-NOx Omnibus and Advanced Clean Cars II emissions rules.

California believes that in taking the unprecedented action to revoke the emissions waivers that Congress unconstitutionally exceeded its powers and is challenging it in court.

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However, while that litigation is being determined, the state wanted to make sure manufacturers can still sell vehicles and engines into California. 

'Unprecedented Degree of Uncertainty' in California New-Vehicle Market

“The congressional resolutions have introduced an unprecedented degree of uncertainty into the California market for new motor vehicles,” the board said in its regulatory notice. Because the emissions regulations that were invalidated by Congress had superseded previous emissions regulations, that has left questions about which regulations apply.

These emergency amendments are reinstating the emissions regulations that were in effect prior to the Low-NOx Omnibus and Advanced Clean Cars II rules (LEV IV).

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In the case of the Advanced Clean Cars II rules, CARB said, this means the criteria pollution provisions of the LEV III regulation (adopted as part of ACC I) and associated on-board diagnostic requirements remain operative.

The new emergency amendments include the caveat that CARB may enforce the Omnibus and Advanced Clean Cars II regulations, to the extent permitted by law, if it wins its challenge in court.

Are Any CARB Standards Still Legal?

In a court filing on September 4, CARB said, vehicle manufacturers argued that CARB’s earlier-adopted vehicle certification requirements — for which CARB received separate earlier waivers not affected by the congressional resolutions  — are invalid because those requirements became “defunct” when the more recent, more stringent standards displaced them.

“Otherwise, in light of these unprecedented circumstances, there may remain questions — for the first time since CARB’s program began decades ago — as to whether any California standard is in effect.”

Thus, according to the vehicle manufacturer challengers, “amending state law” would be required to revive the earlier adopted standards even if the displacing standards are ultimately found to be unenforceable on account of Congress’s actions.

CARB disagrees. 

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"Nevertheless, in the event the vehicle manufacturer’s claim were to be deemed correct — that CARB must revive its earlier-adopted emissions requirements before it can resume enforcement of them — then CARB must take immediate action to maintain a stable vehicle market in the state and prevent the sale of vehicles into the state that would not be certified to either set of standards," it explained.

“Otherwise, in light of these unprecedented circumstances, there may remain questions — for the first time since CARB’s program began decades ago — as to whether any California standard is in effect.”

“This emergency regulation thus responds to an unprecedented and unanticipated set of circumstances: the suggestion by a regulated party that, for the first time in half a century, CARB has no operative emissions standards to which it can certify new motor vehicles to be sold in the state.”

Both Sets of Emissions Standards Will Be On the Books

CARB continues to accept and process certification applications for the LEV IV and Omnibus emission standards. 

“Hence, both sets of standards will be present in the California Code of Regulations during this period of unprecedented uncertainty,” the agency explained in its regulatory notice.

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“Regulated parties may choose to follow either the LEV IV or Omnibus standards or the antecedent LEV III and pre-Omnibus provisions. 

“Regulated parties, however, assume the risk if they choose to certify only to the antecedent provisions, and the congressional resolutions disapproving the waivers of federal preemption under the Clean Air Act are declared invalid.”

CARB intends to submit the Emergency Vehicle Emissions Regulation to OAL on September 22. The submitted action will appear on the list of “Emergency Regulations Under Review” on OAL’s website at: https://oal.ca.gov/emergency_regulations/Emergency_Regulations_Under_Review/

Emergency rulemakings have a five-calendar-day comment period that begins when OAL posts the notice of the pending emergency action on the OAL website.

What About ZEVs?

The emergency rules do not appear to include California’s zero-emission truck regulations. Since the Congressional resolutions revoking the EPA waivers were signed in June, California has maintained that heavy-duty truck and engine makers were still bound to meet the Low-NOx and Advanced Clean Trucks standards under the Clean Truck Partnership. 

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But there has been pushback, with truck makers suing California over its attempt to still enforce the clean truck rules, and the Federal Trade Commission raising antitrust concerns regarding the Clean Truck Partnership.

California’s History of Regulating Truck Emissions

California first regulated heavy-duty vehicle exhaust emissions in 1969. 

In 2005 and 2010, U.S. EPA granted waivers for California’s medium- and heavy-duty engine and vehicle regulations for diesel and Otto-cycle engine standards. 

In 2016, U.S. EPA also granted California a waiver for its Phase 1 Greenhouse Gas emission standards adopted in 2013.

In 2016, the U.S. EPA granted California’s requests for waivers for its on-board diagnostic (OBD) systems for light-, medium-, and heavy-duty vehicles adopted in 2013 (the OBD II and HD OBD regulations).

In 2017, the U.S. EPA granted California a waiver of federal preemption for several sets of amendments to CARB’s emission standards for medium- and heavy-duty vehicles adopted in 2011, 2008, 2007, and 2006.

California’s Low-NOx Omnibus regulations, adopted in 2020 and amended in 2023, tightened CARB’s criteria pollutant standards for medium- and heavy-duty vehicles. 

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