Cal. Civ. Proc. Code § 1297.114

If the appointment procedure in Section 1297.113 applies and either a party fails to appoint an arbitrator within 30 days after receipt of a request to do so from the other party, or the two appointed arbitrators fail to agree on the third arbitrator within 30 days after their appointment, the appointment shall be made, upon request of a party, by the superior court.