California Code, Probate Code - PROB § 9254

(a) The validity of an allowed or approved claim may be contested by any interested person at any time before settlement of the report or account of the personal representative in which it is first reported as an allowed or approved claim. The burden of proof is on the contestant, except where the personal representative has acted under the Independent Administration of Estates Act (Part 6 (commencing Section 10400)), in which case the burden of proof is on the personal representative.

(b) Subdivision (a) does not apply to a claim established by a judgment.