Cal. Bus. & Prof. Code § 9884.10

Upon request of the customer at the time the work order is taken, the automotive repair dealer shall return replaced parts to the customer at the time of the completion of the work excepting such parts as may be exempt because of size, weight, or other similar factors from this requirement by regulations of the department and excepting such parts as the automotive repair dealer is required to return to the manufacturer or distributor under a warranty arrangement. If such parts must be returned to the manufacturer or distributor, the dealer at the time the work order is taken shall offer to show, and upon acceptance of such offer or request shall show, such parts to the customer upon completion of the work, except that the dealer shall not be required to show a replaced part when no charge is being made for the replacement part.