California Code, Probate Code - PROB § 8004

(a) If appointment of the personal representative is contested, the grounds of opposition may include a challenge to the competency of the personal representative or the right to appointment.  If the contest asserts the right of another person to appointment as personal representative, the contestant shall also file a petition and serve notice in the manner provided in Article 2 (commencing with Section 8110) of Chapter 2, and the court shall hear the two petitions together.

(b) If a will is contested, the applicable procedure is that provided in Article 3 (commencing with Section 8250) of Chapter 3.