Cal. Veh. Code § 27157

The State Air Resources Board, after consultation with, and pursuant to the recommendations of, the commissioner, shall adopt such reasonable regulations as it determines are necessary for the public health and safety regarding the maximum allowable emissions of pollutants from vehicles upon a highway. Such regulations shall apply only to vehicles required by Part 5 (commencing with Section 43000) of Division 26 of the Health and Safety Code or any federal law or regulation to be equipped with devices or systems to control emission of pollutants from the exhaust and shall not be stricter than the emission standards required of that model year motor vehicle when first manufactured.