California Code, Commercial Code - COM § 2801

In any retail sale of goods, if the manufacturer or seller of the goods issues a written warranty or guarantee as to the condition or quality of all or part of the goods which requires the buyer to complete and return any form to the manufacturer or seller as proof of the purchase of the goods, such warranty or guarantee shall not be unenforceable solely because the buyer fails to complete or return the form. This section does not relieve the buyer from proving the fact of purchase and the date thereof in any case in which such a fact is in issue.

The buyer must agree in writing to any waiver of this section for the waiver to be valid. Any waiver by the buyer of the provisions of this section which is not in writing is contrary to public policy and shall be unenforceable and void.