Vehicle Code § 38395

This article shall not apply to an alteration, modification, or modifying device, apparatus, or mechanism found by resolution of the State Air Resources Board either:

(a) To not reduce the effectiveness of any required off-highway motor vehicle pollution control device; or

(b) To result in emissions from any such modified or altered off-highway vehicle which are at levels which comply with existing state or federal standards for that model year of the vehicle being modified or converted.