California Code, Probate Code - PROB § 8400

(a) A person has no power to administer the estate until the person is appointed personal representative and the appointment becomes effective. Appointment of a personal representative becomes effective when the person appointed is issued letters.

(b) Subdivision (a) applies whether or not the person is named executor in the decedent's will, except that a person named executor in the decedent's will may, before the appointment is made or becomes effective, pay funeral expenses and take necessary measures for the maintenance and preservation of the estate.

(c) The order appointing a personal representative shall state in capital letters on the first page of the order, in at least 12-point type, the following: “WARNING: THIS APPOINTMENT IS NOT EFFECTIVE UNTIL LETTERS HAVE ISSUED.”