California Code, Probate Code - PROB § 4230

(a) Except as provided in subdivisions (b) and (c), a person who is designated as an attorney-in-fact has no duty to exercise the authority granted in the power of attorney and is not subject to the other duties of an attorney-in-fact, regardless of whether the principal has become incapacitated, is missing, or is otherwise unable to act.

(b) Acting for the principal in one or more transactions does not obligate an attorney-in-fact to act for the principal in a subsequent transaction, but the attorney-in-fact has a duty to complete a transaction that the attorney-in-fact has commenced.

(c) If an attorney-in-fact has expressly agreed in writing to act for the principal, the attorney-in-fact has a duty to act pursuant to the terms of the agreement. The agreement to act on behalf of the principal is enforceable against the attorney-in-fact as a fiduciary regardless of whether there is any consideration to support a contractual obligation.