As a ground for vacating an award, C.C.P. 1286.2 [a]) includes an arbitrator's "refusal to hear evidence material to the controversy," causing "substantial prejudice" to the parties; Flanagan v. Orbison, 2005 WL 2224836 (Cal. App.). The court must accept the arbitrator's legal theory and conclude the arbitrator might have rendered a different award having excluded the evidence; Hall v. Sup. Ct., 18 Cal. App. 4th 427 (1993); Gonzales v. Interinsurance Exchange, 84 Cal. App. 4th 58 (1978). Without citing either of these cases, the court in Burlage v. Sup. Ct., 2009 WL 2712382 (Cal.App.) vacated an award on the ground (apparently) the court majority disliked the arbitration award despite two California Supreme Court cases holding arbitrator legal error is not subject to review; Cable Connection, Inc. v. DIRECTV, Inc., 44 Cal. 4th 1334 (2008); Moncharsh v. Heily and Blasé, 3 Cal. 4th 1 (1992).
Failure to "hear material evidence", literally interpreted, would engender an endless round of appeals if reviewed in litigation terms of "materiality." The ground, if established, requires substantial prejudice; Schlessinger v. Rosenfeld, et al., 40 Cal. App. 4th 1096 (1995). CCP 1286.2 is "a safety valve in arbitration that permits a court to intercede when an arbitrator has prevented a party from fairly presenting its case;" Schlessinger; Hall v. Sup. Ct.
In Kaiser Foundation Health Plan, Inc. v. Ayon Alexandre, 2006 WL 552631 (Cal. App.) [Non. Cite.] the court interprets CCP 1286.2 [a]  to require evidence of prejudice first, not materiality.
See, Cable Connection, Inc. v. DIRECTV, 143 Cal. App. 4th 207 (2006) (distinguishing the failure to "hear" material evidence and "consider" material evidence), reversed on other grounds); Gonzales v. Interinsurance Exchange.