Cal. Rev. & Tax. Code § 3456
(a) If any part of a bid that was accepted by the tax collector is not paid when due, the county shall have a claim against the bidder for the actual cost of the sale. Immediately upon becoming past due, the tax collector shall notify the bidder by certified mail, which notice shall include all of the following:
(b) If a bidder does not pay the claim arising under subdivision (a) within 30 days after the notice is sent, both of the following apply: