California Code, Vehicle Code - VEH § 38366

(a) Notwithstanding Section 4442 of the Public Resources Code, and except for vehicles with mufflers as provided in Article 2 (commencing with Section 27150) of Chapter 5 of Division 12, no person shall use, operate, or allow to be used or operated, any off-highway motor vehicle, as defined in Section 38006, on any forest-covered land, brush-covered land, or grass-covered land unless the vehicle is equipped with a spark arrester maintained in effective working order.

(b) A spark arrester affixed to the exhaust system of a vehicle subject to this section shall not be placed or mounted in such a manner as to allow flames or heat from the exhaust system to ignite any flammable material.

(c) A spark arrester is a device constructed of nonflammable materials specifically for the purpose of removing and retaining carbon and other flammable particles over 0.0232 of an inch in size from the exhaust flow of an internal combustion engine or which is qualified and rated by the United States Forest Service.

(d) Subdivision (a) shall not be applicable to vehicles being operated off the highway in an organized racing or competitive event upon a closed course, which is conducted under the auspices of a recognized sanctioning body and by permit issued by the fire protection authority having jurisdiction.