Cal. Veh. Code § 4000.15

(a) Effective January 1, 2020, the department shall confirm, prior to the initial registration or the transfer of ownership and registration of a diesel-fueled vehicle with a gross vehicle weight rating of more than 14,000 pounds, that the vehicle is compliant with, or exempt from, applicable air pollution control technology requirements pursuant to Division 26 (commencing with Section 39000) of the Health and Safety Code and regulations of the State Air Resources Board adopted pursuant to that division.
(b) Except as otherwise provided in subdivision (c), for diesel-fueled vehicles subject to Section 43018 of the Health and Safety Code, as applied to the reduction of emissions of diesel particulate matter, oxides of nitrogen, and other criteria pollutants from in-use diesel-fueled vehicles, and Section 2025 of Title 13 of the California Code of Regulations as it read January 1, 2017, or as subsequently amended:
1. (1) The department shall refuse registration, or renewal or transfer of registration, for a diesel-fueled vehicle with a gross vehicle weight rating of 14,001 pounds to 26,000 pounds for the following vehicle model years:
  1. (A) Effective January 1, 2020, vehicle model years 2004 and older.
  2. (B) Effective January 1, 2021, vehicle model years 2007 and older.
  3. (C) Effective January 1, 2023, vehicle model years 2010 and older.
2. (2) The department shall refuse registration, or renewal or transfer of registration, for a diesel-fueled vehicle with a gross vehicle weight rating of more than 26,000 pounds for the following vehicle model years:
  1. (A) Effective January 1, 2020, vehicle model years 2000 and older.
  2. (B) Effective January 1, 2021, vehicle model years 2005 and older.
  3. (C) Effective January 1, 2022, vehicle model years 2007 and older.
  4. (D) Effective January 1, 2023, vehicle model years 2010 and older.
(c)
1. (1) As determined by the State Air Resources Board, notwithstanding effective dates and vehicle model years identified in subdivision (b), the department may allow registration, or renewal or transfer of registration, for a diesel-fueled vehicle that has been reported to the State Air Resources Board, and is using an approved exemption, or is compliant with applicable air pollution control technology requirements pursuant to Division 26 (commencing with Section 39000) of the Health and Safety Code and regulations of the State Air Resources Board adopted pursuant to that division, including vehicles equipped with the required model year emissions equivalent engine or otherwise using an approved compliance option.
2. (2) The State Air Resources Board shall notify the department of the vehicles allowed to be registered pursuant to this subdivision.