California Air Resources Board

This fact sheet is developed to provide general information about the Omnibus regulation.

Why does California need to further reduce emissions from on-road heavy-duty vehicles and engines?

In 2023, heavy-duty vehicles were the largest contributors of oxides of nitrogen (NOx) emissions in California. NOx emissions lead to dangerous ozone and fine particulate matter (PM2.5) formation in the atmosphere.

2023 Statewide NOx Emissions Inventory*

*Data was derived from CEPAM2019v1.03 - Standard Emission Tool.

Therefore, further NOx reductions are needed from heavy-duty vehicles to achieve California's air quality goals. The Omnibus regulation is a major regulation designed to significantly reduce NOx emissions from heavy-duty vehicles and is vital for achieving California’s State Implementation Plan (SIP) commitments to attain the federal standards for ozone and PM2.5 in 2031.

What is the Omnibus regulation?

The Omnibus regulation was adopted by the Board in September 2021, and amended in December 2023. The primary goal of the regulation is to reduce NOx emissions from medium‑ and heavy‑duty engines and vehicles beginning in 2024 model year (MY). Highlights of the regulation include:

• Lower NOx and particulate matter (PM) emissions standards as well as a new NOx emission standard on the new low‑load certification cycle. The Omnibus NOx emission standard for 2024 through 2026 MY engines represents a 75% reduction of the 2010 MY standard, from 0.20 to 0.050 grams per brake horsepower hour (g/bhp-hr). In addition, a lower PM standard of 0.005 g/bhp-hr, that is 50% lower than the 2010 MY emission standard, was established. More stringent NOx emission standards were also established for 2027 and subsequent MYs;
• A revamping of the heavy-duty in-use testing program; 
• Enhancing the Warranty period, Useful Life, and Emissions Warranty Information and Reporting; and
• Strengthening the heavy-duty durability demonstration program.

Why was the regulation amended in 2023?

In 2023, as the timeline for manufacturers to submit certification applications for 2024 MY engines was drawing near, California Air Resources Board (CARB) staff received information regarding changes to heavy-duty engine distribution plans for 2024 to 2026 MY period. While the technology needed for diesel‑fueled engines to comply with the Omnibus regulation was available at that time, some manufacturers decided to not produce California-compliant engines for some categories of trucks. Given the impacts to the fleets, additional flexibility was desired to maintain the originally anticipated engine supplies for 2024 through 2026 MYs, and to enable a smoother transition to the Omnibus standards.

CARB considered and ultimately adopted amendments to the Omnibus regulation in order to provide the manufacturers with additional flexibility, and to prevent product availability issues for the 2024 through 2026 MY period, while maintaining the overall emission benefits of the program.

Does the regulation provide compliance flexibilities?

The latest Omnibus amendments contain several flexibilities for engine manufacturers to help manage the transition to these new standards. These include:

• The Omnibus regulation contains an optional provision that allows 2024 through 2026 MY heavy-duty engines rated at or above 525 brake-horsepower to be certified to the applicable federal certification requirements, except that the engine must also comply with the California 30 grams per hour NOx idling standard and the applicable California Step 1 emission warranty requirements;
• The Omnibus regulation also includes a provision that allows transit agencies subject to the Innovative Clean Transit regulation to request an exemption to procure diesel-fueled transit buses that are certified to the 2010 MY federal emissions standards. If an exemption request is granted, the transit agency can purchase or re-power buses with engines that are certified to the 2010 MY federal emission standards; and
• Finally, the Omnibus regulation includes the legacy engine provisions that allow limited production of 2024-2026 MY heavy-duty engines rated below 525 brake-horsepower, referred to as legacy engines. Legacy engines are only required to meet the 2010 MY NOx and PM emissions standards. Legacy engine sales in California are only allowed if the manufacturer offsets any excess NOx and PM emissions deficits through the use of heavy‑duty zero‑emission powertrain credits, heavy-duty combustion credits, or performing projects in disadvantaged communities. There are two certification pathways for legacy engines in California:
  • Option 1 - Legacy engine sales limits of 45% for MY 2024, 25% for MY 2025, and 10% for MY 2026; or
  • Option 2 - Applicable to manufacturers that certify and sell both medium heavy-duty engines and engines in other primary intended service classes: 60% medium heavy‑duty engines in 2024 and 2025 MYs, and 15% heavy-duty engines in other primary intended service classes in MY 2024, and 8% in MY 2025. No legacy engine sales are allowed in MY 2026.

To qualify for the legacy engine provisions, a manufacturer must certify at least one engine family to the Omnibus requirements.

What emission reductions would the regulation achieve?

The Omnibus regulation was the single largest NOx control measure in the 2016 State SIP Strategy, and was estimated to reduce statewide NOx emissions in California by approximately 17.4 tons per day (tpd) in 2031, and 45.2 tpd, in 2050. The 2023 amendments did not affect the overall emissions benefits of the program, but provided the manufacturers with additional compliance flexibility during the 2024 through the 2026 MY period.

What is the Clean Truck Partnership agreement?

On July 5, 2023, CARB reached an agreement with the Truck and Engine Manufacturers Association (EMA), members of EMA that manufacture heavy-duty on-road vehicles and engines, and the Ford Motor Company. That agreement is also known as the Clean Truck Partnership (Partnership). The Partnership required CARB staff to propose amendments to the Omnibus regulation legacy engine provisions to provide greater flexibility in meeting the Omnibus requirements for the 2024 to 2026 MY period. This commitment was subsequently fulfilled through the Omnibus amendments in 2023. The Partnership also specified, in pertinent part, that CARB staff commits to propose amendments to the Omnibus regulation that will mostly align the emissions standards and the test procedures for 2027 and subsequent MY heavy-duty engines with the corresponding provisions in the United States Environmental Protection Agency’s (U.S. EPA) Clean Trucks Plan (CTP) NOx rule. CARB staff subsequently held a public workshop regarding the proposed harmonization with the CTP-NOx rule.

How would this regulation interact with CARB’s Advanced Clean Trucks Regulation?

The Advanced Clean Trucks (ACT) Regulation, adopted in June 2020, is a central element of CARB's strategy to reduce harmful emissions from medium- and heavy-duty vehicles to the greatest degree feasible. The ACT regulation mandates percentage‑of‑production zero‑emission truck sales in California to broaden the market and to send a clear signal that medium- and heavy-duty ZEVs will be a major part of California’s overall strategy to reduce criteria emissions, reduce climate impacts, and reduce petroleum use.

The Omnibus regulation is intended to achieve NOx reductions from the trucking sector in the short term, while the ACT regulation is intended to help California achieve its long term decarbonization goals by mandating the electrification of the heavy-duty sector. The Omnibus regulation will complement the ACT regulation by ensuring that all heavy-duty internal combustion engines will emit the lowest possible NOx emission levels that are technologically possible.

What are the solutions to address product availability issues?

Amendments to both the Omnibus and ACT regulations have provided manufacturers with more compliance flexibilities intended to remedy product availability issues in the California market. CARB staff also developed a memo with respect to California Truck Availability Analysis, and also issued several enforcement discretion letters to further alleviate any product availability issues.

Where can I find more information about the regulation?

Regulatory documents, resources, and information on meetings/workshops are available at Heavy-Duty Low NOx. For questions, send an email at lownox@arb.ca.gov.

How will I get updates about the regulation?

Subscribe at hdlownox to keep up to date with the latest information regarding the Omnibus regulation.