California Code, Health and Safety Code - HSC § 43108

(a) In lieu of certification pursuant to Section 43102, the state board may certify a new motor vehicle designed for exclusive use as a schoolbus, or a new motor vehicle engine intended for use in a schoolbus, if the Administrator of the Environmental Protection Agency has granted a certificate of conformity for the schoolbus or engine pursuant to the Clean Air Act (42 U.S.C. Sec. 1857 et seq.)  1.

(b) The state board shall grant a certification pursuant to subdivision (a) only if the manufacturer of the schoolbus or engine demonstrates that an engine suitable for use in the manufacturer's standard type of schoolbus which meets the applicable emissions standards established by the state board pursuant to Section 43102 is not available for installation.

(c) The state board, prior to granting a certification pursuant to subdivision (a), shall require a showing by the manufacturer of the schoolbus or engine of a good faith effort to procure or manufacture an engine which meets the standards established by the state board pursuant to Section 43102 and, in the case of the schoolbus manufacturer, a good faith effort to accomplish a schoolbus redesign to accommodate such an engine. In the absence of these showings, the state board shall not grant a certification pursuant to subdivision (a).