Carmakers are lobbying against an extraordinary push by U.S. regulators to force a recall of as many as 52 million air-bag inflators.

Companies like General Motors and Ford Motor said in letters to the National Highway Traffic Safety Administration released Wednesday that there¡¯s no proof parts made by ARC Automotive are defective, despite reports that some inflators have exploded in a crash, injuring or killing passengers. NHTSA is calling for a broad recall over ARC¡¯s objections ¡ª a move that may cost carmakers up to $10 billion.

Despite objections from ARC and multiple automakers, NHTSA is continuing with the steps required for it to take unilateral action. The comment period ended Monday, and the agency will make a final decision on whether the parts are defective as early as next year, according to a person familiar with its thinking. That would be the last step before a mandatory recall, although ARC could challenge the decision in court.