Workgroup meeting to discuss Tier 5 rulemaking concepts on ABT, Idle reduction, and OBD

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November 10, 2022

Public Workgroup Meeting on December 14, 2022, to Discuss the Off-Road New Diesel Engine Tier 5 Rulemaking: Concepts for Averaging, Banking, and Trading, Idle Reduction, and On-Board Diagnostics.

The California Air Resources Board (CARB) staff invites you to participate remotely in a public workgroup meeting on December 14, 2022, 8:30 AM - 5:00 PM to discuss possible concepts and strategies to reduce oxides of nitrogen (NOx) and particulate matter (PM) emissions from new, off-road compression-ignition (CI) engines.

During the workgroup meeting, CARB staff will discuss concepts for the off-road new diesel engine Tier 5 rulemaking including a California-specific Averaging, Banking, and Trading (ABT) program, idle reduction, and On-Board Diagnostics (OBD).

The current ABT program is administered federally and allows engine manufacturers flexibility in complying with the standards by averaging emission levels across power categories and product lines nationwide. Significantly more stringent off-road diesel standards in California could result in excessive credit usage in California, potentially delaying the introduction of engines meeting Tier 5 standards indefinitely. To prevent this, California is considering adopting statewide averaging requirements to ensure that credits are used equitably within California and that Tier 5 compliant engines enter the California market in a timely manner.

CARB staff will also discuss concepts for reducing emission from off-road diesel engine idling. Currently, although California’s In-Use Off-Road Diesel-Fueled Fleets Regulation contains a 5-minute idle restriction and a requirement for affected fleets to have a written idle policy, that regulation does not apply to all diesel engines nor to all idling, and California’s new engine standards do not include idling restrictions. Prolonged idling can adversely affect exhaust temperature and Selective Catalytic Reduction (SCR) function resulting in higher NOx emissions. Off-road diesel engines can operate at idle for up to 50 percent of the time. The potential idle reduction concepts include requiring use of idle control systems such as auto shutdown or introducing an idling standard.

OBD is  currently not required for off-road engines. Many OBD features from existing on-road heavy-duty diesel engines could be applied to off-road engines. Staff will introduce some potential OBD concepts for off-road diesel engines.

The workgroup meeting will be held via Zoom webinar at the following date and time:

Date:                 December 14, 2022
Time:                 8:30 a.m. – 5:00 p.m. PST
Location:           Zoom Webinar/Teleconference


After registering, you will receive a confirmation email containing information about joining the workgroup meeting. Please use the link and password to join the workgroup the morning of the meeting. If you are using a tablet or smart phone, please download the Zoom app. Ensure the latest version of Zoom is installed on your device.

During the workgroup meeting, if you wish to provide comments or ask questions on the potential regulatory concepts, please use Zoom’s “raise hand” feature. The “raise hand” button can be found on the Zoom webinar window. For those who are joining via a conference call, press #2 to raise your hand. The “Q&A” feature on Zoom will also be available to provide comments and ask questions. This workgroup meeting will be recorded for future viewing.

Staff’s presentation, and any additional webinar related documents, will be made available approximately one week prior to the workgroup meeting on CARB’s website.


Currently, CARB’s regulations for new off-road diesel engines are largely harmonized with the U.S. Environmental Protection Agency’s for nonroad diesel engines. Off-road and nonroad refer to the same type of engine or equipment with off-road being used exclusively in California. The engines in this sector are land-based and range from less than 19 kiloWatt (kW) to well over 560 kW in rated power. The existing Tier 4 emission standards were adopted in 2004 and were based on the capabilities of the best available technologies at that time: diesel particulate filters (DPF) and SCR. As of model year 2020, more than half of all new off-road CI engine families continue to be certified in California to the Tier 4 final emission standards without DPFs. Exhaust aftertreatment technology has matured significantly in the last 18 years. Significant reductions in PM and NOx  are now feasible beyond the current Tier 4 standards of 0.02 grams per kilowatt-hour (g/kW‑hr) and 0.40 g/kW-hr for 56 – 560 kW engines, respectively.

CARB staff is working on potential amendments to the current Tier 4 off-road diesel engine standards for the Tier 5 rulemaking. Staff will likely propose more stringent exhaust emission standards for NOx (up to 90 percent more stringent) and PM (up to 75 percent more stringent) than today’s Tier 4 standards. First-time tailpipe carbon dioxide (CO2) emission standards for off-road engines will also likely be proposed by CARB. CARB staff is considering proposing possible elements such as enhancing in-use compliance testing, proposing more representative useful-life periods, and developing a low-load test cycle. CARB staff will also propose to update the test procedures and consider first-time off-road OBD requirements. CARB staff’s proposal is tentatively scheduled for Board consideration in the 2025 timeframe, with implementation beginning as early as the 2028-2029 model years.

CARB is currently contracting with the Southwest Research Institute (SwRI) to investigate the feasibility of standards lower than Tier 4 standards. SwRI is working on assessing the feasibility of lower NOx , PM, and CO2 emission standards.

This workgroup meeting is the fourth public event in CARB’s Tier 5 rulemaking. CARB staff invites participation from industry, fleet owners, trade associations, non-governmental organization representatives, environmental groups, air districts, and other interested stakeholders. CARB staff invites participants to provide feedback on the potential concepts presented at the workgroup meeting.


If you have questions, contact

If you require a special accommodation or need this document in another language or in an alternate format (i.e., Braille, large print), please contact Isabel Guerra as soon as possible, but no later than 15 business days before the scheduled webinar. TTY/TDD/Speech to Speech users may dial 711 for the California Relay Service.

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