What is the test for vacating an arbitrator's award?

"[A]n arbitrator exceeds its powers within the meaning of Code of Civil Procedure section 1286.2 by issuing an award that violates a party's statutory rights or 'an explicit legislative expression of public policy.' [Citations.] . . . [] Therefore, 'courts may, indeed must, vacate an arbitrator's award when it violates a party's statutory rights or otherwise violates a well-defined public policy.' [Citation.] . . . [] This exception is applicable only when there has been '"a clear expression of illegality or public policy"' that undermines the presumption in favor of private arbitration." (Ahdout v. Hekmatjah (2013) 213 Cal.App.4th 21, 37-38.)