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 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.