Cal. VEH § 17004.5

Any private firm or corporation, or employee thereof, which maintains a fire department and has entered into a mutual aid agreement pursuant to Section 13855, 14095, or 14455.5 of the Health and Safety Code shall have the same immunity from liability for civil damages on account of personal injury to or death of any person or damage to property resulting from the operation of an authorized emergency vehicle while responding to, but not upon returning from, a fire alarm or other emergency call as is provided by law for the district and its employees with which the firm or corporation has entered into a mutual aid agreement, except when the act or omission causing the personal injury to or death of any person or damage to property occurs on property under the control of such firm or corporation.