Transportation

Low NOx Omnibus Rules

The Low NOx Omnibus Rules, pioneered in California, represent a comprehensive set of regulations designed to significantly curb nitrogen oxide (NOx) emissions from on-road heavy-duty vehicles. These rules encompass stringent tailpipe emission standards, updated testing procedures, and technology-neutral compliance mechanisms to achieve substantial reductions in NOx pollution.

In the California context, NOx emission limits for heavy-duty vehicles are progressively tightened over time, with targeted reductions of up to 90% by 2031 relative to expected emissions without the regulation in place. Manufacturers are required to adhere to revised emission limits starting from model year 2024, with further updates in subsequent model years.

A key aspect of the Low NOx Omnibus Rules is their technology-neutral approach, providing manufacturers the flexibility to select both engine and aftertreatment technologies to meet strict emission standards. While engine adjustments may help reduce NOx emissions slightly, the primary focus for emission control is on advancements in aftertreatment processes, such as selective catalytic reduction (SCR) or diesel particulate filters (DPF), which are crucial for significant emission reductions.

In June 2025, President Trump signed three Congressional Review Act (CRA) resolutions repealing California’s Clean Air Act waivers for Advanced Clean Cars II, Advanced Clean Trucks, and the Low NOx Omnibus Rules. A coalition of states that have adopted the rules filed a lawsuit against the federal government in response.

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Glossary of Terms

States with Policy Enacted

In Progress

Partially Enacted

See States List

Key Resources

Glossary of Terms

Clean Vehicle Programs: State Tracker

A policy tracker that identifies which states have adopted light-duty and medium- and heavy-duty clean vehicle rules, with links to enacted policy text and and information on states' share of new vehicles sold in the U.S.

2024

Source:

Sierra Club

California’s Heavy-Duty Omnibus Regulation: Updates To Emission Standards, Testing Requirements, And Compliance Procedures

A policy update that outlines the inaugural CARB Low NOx Omnibus regulation and its recent developments.

2022

Source:

International Council on Clean Transportation

How to Identify Low NOx Certified Engines

A fact sheet that provides essential guidance for identifying engines certified by the California Air Resources Board (CARB) to meet low NOx standards.

2020

Source:

U.S. EPA

Model Rules

Glossary of Terms

Heavy-Duty Low NOx Omnibus Regulation

Updates emission standards, testing requirements, and compliance procedures for heavy-duty vehicles from model years 2024 to 2031. It aims to reduce NOx emissions by 90% by 2031, with a 30% reduction in overall heavy-duty NOx emissions by 2050 relative to expected emissions without the regulation in place.

2020

Source:

California Air Resources Board

Model States

Glossary of Terms

California

The Low NOx Omnibus Rule applies to Model Year 2024 onwards.

In response to the passage of federal resolutions repealing California’s Clean Air Act waivers for Advanced Clean Cars II (ACC II), Advanced Clean Trucks (ACT), and the Low NOx Omnibus Rules, Governor Newsom directed CARB to propose "Advanced Clean Cars III" regulations as an alternative for ACC II, ACT, and Low NOx rules.

2020

Establishing Policies

Massachusetts

The Low NOx Omnibus Rule applies to Model Year 2026 onwards.

2024

Establishing Policies

New York

The Low NOx Omnibus Rule applies to Model Year 2026 onwards.

2022

Establishing Policies

6 NYCRR Part 218, Emission Standards for Motor Vehicles and Motor Vehicle Engines (2022)

Washington

The Low NOx Omnibus Rule applies to Model Year 2026 onwards.

2022

Establishing Policies

Chapter 173-423 WAC (2022)

Glossary of Terms

Filters

Status	State Sort descending	Region	Year Enacted
Not Enacted	Alabama Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Alaska Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West
Not Enacted	Arizona Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southwest
Not Enacted	Arkansas Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Enacted	California Empty column The Low NOx Omnibus Rule applies to Model Year 2024 onwards. In response to the passage of federal resolutions repealing California’s Clean Air Act waivers for Advanced Clean Cars II (ACC II), Advanced Clean Trucks (ACT), and the Low NOx Omnibus Rules, Governor Newsom directed CARB to propose "Advanced Clean Cars III" regulations as an alternative for ACC II, ACT, and Low NOx rules. Establishing Policies Heavy-Duty Engine and Vehicle Omnibus Regulation (2020) Executive Order N-27-25 (2025) Empty column	West	2020
Enacted	Colorado Empty column The Low NOx Omnibus Rule applies to Model Year 2027 onwards. Establishing Policies Heavy-Duty Low NOx Regulation (2023) Empty column	West	2023
Not Enacted	Connecticut Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Northeast
Not Enacted	Delaware Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Florida Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Georgia Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Hawai‘i Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West
Not Enacted	Idaho Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West
Not Enacted	Illinois Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Indiana Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Iowa Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Kansas Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Kentucky Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Louisiana Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Maine Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Northeast
Not Enacted	Maryland Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Enacted	Massachusetts Empty column The Low NOx Omnibus Rule applies to Model Year 2026 onwards. Establishing Policies 310 CMR 7.40, Low Emission Vehicle Program (2021) Emergency Regulations and Limited Enforcement Discretion for Advanced Clean Trucks Regulations (2024) Empty column	Northeast	2024
Not Enacted	Michigan Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Minnesota Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Mississippi Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Missouri Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Montana Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West
Not Enacted	Nebraska Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Nevada Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West
Not Enacted	New Hampshire Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Northeast
Not Enacted	New Jersey Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Northeast
Not Enacted	New Mexico Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southwest
Enacted	New York Empty column The Low NOx Omnibus Rule applies to Model Year 2026 onwards. Establishing Policies 6 NYCRR Part 218, Emission Standards for Motor Vehicles and Motor Vehicle Engines (2022) Empty column	Northeast	2022
Not Enacted	North Carolina Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	North Dakota Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Ohio Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Oklahoma Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southwest
Enacted	Oregon Empty column The Low NOx Omnibus Rule applies to Model Year 2025 onwards. Establishing Policies Administrative Order No. DEQ-17-2021 (2021) Empty column	West	2021
Not Enacted	Pennsylvania Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Northeast
Enacted	Rhode Island Empty column The Low NOx Omnibus Rule applies to Model Year 2027 onwards. Establishing Policies Low-NOx Heavy Duty Omnibus (HDO) (2023) Empty column	Northeast	2023
Not Enacted	South Carolina Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	South Dakota Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Tennessee Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Texas Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southwest
Not Enacted	Utah Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West
Enacted	Vermont Empty column The Low NOx Omnibus Rule applies to Model Year 2026 onwards. Establishing Policies Heavy-Duty Engine and Vehicle Omnibus Regulation (2022) Empty column	Northeast	2022
Not Enacted	Virginia Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Enacted	Washington Empty column The Low NOx Omnibus Rule applies to Model Year 2026 onwards. Establishing Policies Chapter 173-423 WAC (2022) Empty column	West	2022
Not Enacted	West Virginia Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Southeast
Not Enacted	Wisconsin Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	Midwest
Not Enacted	Wyoming Empty column Low NOx Omnibus Rules establish stringent tailpipe emission standards for heavy duty vehicles, updated testing procedures, and technology-neutral compliance mechanisms to reduce nitrous oxide (NOx) pollution. The regulation must be adopted first by California, and other states may adopt the regulation under the federal Clean Air Act. Empty column	West

Low NOx Omnibus Rules

Key Resources

Model Rules

Model States

Low NOx Omnibus Rules By State