California Code, Insurance Code - INS § 491

The rating plan of a motor vehicle liability insurer shall not provide for an increase in the premium if based upon an accident in which the insured is not at fault, in any manner, as determined by either the accident report or the insurer.  In the event the insurer determines that its insured is at fault contrary to an accident report's specific finding that the insured is not at fault, the insurer shall reach its conclusion only after an investigation.