CARB Implements Heavy-Duty Vehicle Inspection and Maintenance Program

The California Air Resources Board (CARB) is charged with protecting the public from the harmful effects of air pollution and is responsible for developing statewide programs and strategies to reduce pollutants emitted from mobile sources. Heavy-duty vehicles remain one of the largest contributors of smog-forming and toxic emissions. Senate Bill 210 (Leyva, Chapter 298, Statutes of 2019) directs CARB to develop and implement a comprehensive program for testing the emission control systems of heavy-duty diesel and alternative fuel vehicles operating in California. CARB approved the Heavy-Duty Inspection and Maintenance (HD I/M) program in December 2021, with phased program implementation starting in January 2023. Following are answers to frequently asked questions about the program.

What is the HD I/M program?

Like the emissions testing program for passenger vehicles and lighter trucks, the HD I/M program combines periodic vehicle testing with other emissions monitoring. Additionally, the program expands existing enforcement strategies to ensure that heavy-duty vehicle emission control systems are functioning properly while the vehicle is traveling on California roadways. When fully implemented, the HD I/M program will provide air quality benefits necessary to achieve federal air quality mandates and ensure clean air for California communities.

What type of vehicles are subject to the HD I/M program?

Diesel and alternative fuel heavy-duty trucks and buses with a gross vehicle weight rating over 14,000 pounds that operate in California are subject to the HD I/M program. This includes hybrid trucks and buses, commercial vehicles, privately-owned vehicles, government vehicles, as well as vehicles registered outside of California.

What are the phases of implementation for the HD I/M program?

The first phase of the HD I/M program began in January 2023 with roadside emissions monitoring devices deployed to screen for high-emitting vehicles. During this phase, vehicles flagged as potential high emitters may be required to undergo follow-up compliance testing to ensure they are operating with properly functioning emissions control systems. By 2024, the fully implemented HD I/M program will require heavy-duty vehicles operating in California (including out-of-state vehicles) to undergo compliance testing twice per year.

Who can perform HD I/M compliance testing?

HD I/M compliance testing must be performed by a credentialed HD I/M tester who has completed CARB’s online HD I/M tester training course and passed the accompanying examination.

Where can I get additional HD I/M program information?

For more information on CARB’s HD I/M program, visit

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