DETROIT — U.S. safety regulators threatened fines and legal action against Takata Corp. Wednesday unless the company admits that its driver’s air bag inflators are defective and agrees to a nationwide recall. In a letter to Takata’s Washington office, the National Highway Traffic Safety Administration gave the Japanese company until Tuesday to file paperwork declaring a defect and expanding the recall from high-humidity states to the full nation. Takata has maintained that the air bag problems are caused by prolonged exposure to airborne moisture, and that there’s no need for a national recall. Boundaries of the recall zone vary by manufacturer, but generally it covers Gulf Coast states, Hawaii, Puerto Rico and some U.S.