Staff has posted an errata to correct errors and provide clarity to the Oct. 30-31, 2023, workshop slides, and provided the low load certification test cycle.

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November 15, 2023

Errata to the Two-Day Public Workshop on the Off-Road New Diesel Engine Emission Standards (Tier 5 Rulemaking)

October 30 and 31, 2023

Staff has posted an errata to correct errors and provide additional clarity to the Oct. 30-31, 2023, workshop slides, and provided the low load certification test cycle as a downloadable Excel file.

In the errata slide deck, the edits are shown in red, and a new acronym list has been added.

The California Air Resources Board (CARB) staff held a two-day public workshop on October 30 and 31, 2023, 8:00 am – 5:00 pm, to discuss staff’s latest proposal to reduce criteria pollutants and tailpipe greenhouse gases (GHG) from new off-road compression-ignition (CI) engines (i.e., Tier 5 Rulemaking).

During the workshop, CARB staff introduced proposals including more stringent exhaust standards for all off-road diesel engine power categories, on-board diagnostics (OBD), and in-use programs.

Elements of the Tier 5 proposal presented at the workshop included:

  • Tier 5 implementation timing
  • Criteria pollutant standards
  • GHG pollutant standards
  • Adoption of an off-road Low Load Cycle for certification
  • Reduced idling provisions
  • Revised useful life, warranty, and defects reporting requirements
  • The use of accelerated aging for certification
  • New and enhanced in-use screening and testing provisions
  • California-specific credit provisions
  • Selective Catalytic Reduction (SCR) Inducement provisions
  • Tier 5 OBD
  • Overview of modified off-road inventory methodologies


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 kilowatts (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 particulate matter (PM) and oxides of nitrogen (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 PM and NOx, respectively.

CARB staff is working on potential amendments to the current Tier 4 off-road diesel engine standards for the Tier 5 rulemaking. Staff is proposing 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. CARB staff is also proposing first-time tailpipe GHG emission standards for off-road engines. Staff is also looking at enhancing in-use compliance testing, proposing more representative useful-life periods, and developing a low-load test cycle. CARB staff will also update the test procedures and consider first-time off-road OBD requirements. Staff’s proposal is tentatively scheduled for Board consideration in the 2025 timeframe, with implementation beginning as early as the 2029 model year.

CARB staff is currently contracting with the Southwest Research Institute (SwRI) to demonstrate the feasibility of standards lower than Tier 4 standards. SwRI is assessing the feasibility of lower NOx, and PM standards, as well as the addition of carbon dioxide (CO2), nitrous oxides (N2O) and methane (CH4) tailpipe standards.

CARB staff invites participation from industry, fleet owners, trade associations, non-governmental organization representatives, environmental groups, air districts, and the public. Staff invites participants to provide feedback on the proposals presented at the workshop.



If you have questions, please contact: Tier 5

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