Cal. Code of Civil Procedure § 2033.280

If a party to whom requests for admission are directed fails to serve a timely response, the following rules apply: (a) The party to whom the requests for admission are directed waives any objection to the requests, including one based on privilege or on the protection for work product under Chapter 4 (commencing with Section 2018.010). The court, on motion, may relieve that party from this waiver on its determination that both of the following conditions are satisfied: (1) The party has subsequently served a response that is in substantial compliance with Sections 2033.210, 2033.220, and 2033.230. (2) The party’s failure to serve a timely response was the result of mistake, inadvertence, or excusable neglect. (b) The requesting party may move for an order that the genuineness of any documents and the truth of any matters specified in the requests be deemed admitted, as well as for a monetary sanction under Chapter 7 (commencing with Section 2023.010). (c) The court shall make this order, unless it finds that the party to whom the requests for admission have been directed has served, before the hearing on the motion, a proposed response to the requests for admission that is in substantial compliance with Section 2033.220. It is mandatory that the court impose a monetary sanction under Chapter 7 (commencing with Section 2023.010) on the party or attorney, or both, whose failure to serve a timely response to requests for admission necessitated this motion.