Cal. Lab. Code § 6601.5

If, within 15 working days from receipt of a special order, or action order by the division, the employer fails to notify the appeals board that he or she intends to contest the order, and no notice contesting the abatement period is filed by any employee or representative of the employee within that time, the order shall be deemed a final order of the appeals board and not subject to review by any court or agency. The 15-day period may be extended by the appeals board for good cause.