California Code, Labor Code - LAB § 5506

If the defendant fails to appear or answer, no default shall be taken against him, but the appeals board shall proceed to the hearing of the matter upon the terms and conditions which it deems proper. A defendant failing to appear or answer, or subsequently contending that no service was made upon him, or claiming to be aggrieved in any other manner by want of notice of the pendency of the proceedings, may apply to the appeals board for relief substantially in accordance with the provisions of Section 473 of the Code of Civil Procedure. The appeals board may afford such relief. No right to relief, including the claim that the findings and award of the appeals board or judgment entered thereon are void upon their face, shall accrue to such defendant in any court unless prior application is made to the appeals board in accordance with this section. In no event shall any petition to any court be allowed except as prescribed in Sections 5950 and 5951.