Cal. Unemp. Ins. Code § 3264

If any employer or insurer wholly or partially denies liability upon the claim of an employee for disability benefits under an approved plan, the employee may appeal the denial in the manner provided by law and authorized regulations for an appeal on a claim for benefits payable out of the Disability Fund. All decisions of the Appeals Board denying benefits under this section shall be subject to review by the courts of this State by the exclusive remedy of filing a petition for writ of mandate. No such petition may be filed, however, until the employee exhausts the administrative remedies provided for in this division, nor may any other action be commenced by an employee upon a denial of his claim by his employer or insurer, as the case may be, other than that prescribed herein.