The California Supreme Court has let stand a lower court decision that could expose auto makers to billions of dollars of claims in liability suits over alleged air bag and seat belt defects. Previously, the auto industry routinely claimed it could not be sued in state courts for injuries sustained by drivers or passengers who were using restraints that met federal standards. But, in a case last year involving a car imported by Fountain Valley-based Hyundai Motor America, a state appeals court said Hyundai could be sued over a passenger's injuries even though the driver or passenger was using a federally approved seat belt that met all safety standards. Hyundai, the American Automobile Manufacturers Assn. and several other auto makers had asked the high court to suppress the appellate decision--a little-used process that would keep it from being cited in other cases involving automobile product liability. The high court's refusal means the ruling now can be used by lawyers and judges in automobile product liability cases. If widely cited, it "certainly would strengthen the case" against the car makers' main defense in such suits, said UC Berkeley law professor Steven Barnett, a product liability specialist. Executives at Hyundai and the domestic auto makers' trade group in Washington could not be reached for comment.