Cal. Vehicle Code § 28113

(a) Every light-duty and medium-duty motor vehicle operated for compensation to transport persons in an air quality management district or air pollution control district, which does not meet all applicable state ambient air quality standards, shall be a low-emission vehicle, as defined by regulation of the State Air Resources Board. If the vehicle is capable of operating on more than one fuel, it shall be operated within any nonattainment area to the maximum extent practicable either on the designated clean fuel on which the low-emission vehicle was certified or on any other fuel designated by the State Air Resources Board as a substitute fuel for the designated clean fuel. Any air quality management district or air pollution control district may adopt regulations for the enforcement of this section which are consistent with regulations of the State Air Resources Board. (b) As used in this section, “motor vehicle operated for compensation to transport persons” includes a taxi cab, bus, airport shuttle vehicle, transit authority or transit district vehicle, or a vehicle owned by a private entity providing transit service under contract with a transit district or transportation authority. (c) As used in this section, “light-duty” has the same meaning as defined in Section 39035 of the Health and Safety Code. (d) As used in this section, “medium-duty” has the same meaning as defined in Section 39037.5 of the Health and Safety Code. (e) This section applies to all new light-duty motor vehicles purchased on or after January 1, 1997, and to all new medium-duty vehicles purchased on or after January 1, 1998.