General Motors chief Mary Barra has been asked to endorse legislation to keep unrepaired, recalled rental cars off the road.

Although already backed by major rental car firms, the bill has been stalled since 2012. Last week at a Senate Commerce sub committee hearing on GM’s recall of 2.6m vehicles, California Democrat Barbara Boxer questioned Barra about GM’s opposition to the legislation through its industry trade group, the Alliance of Automobile Manufacturers.

The Detroit News noted that Boxer also criticised Barra for declining to answer many questions due to the company’s ongoing investigation into the ignition switch defect linked to at least 13 deaths.

GM has promised to cover the cost of interim rental vehicles while customers await repairs and has already provided more than 16,000 rentals to owners of recalled vehicles.

In a letter to Barra, Boxer has asked for "support for this bill" which is critical as GM customers are renting safe cars while they wait for their recalled vehicles to be repaired.

In May 2013, carmakers came under criticism for opposing a bill that would require rental car companies to park all recalled vehicles until they are repaired. They are worried a proposed law could force them to repair rental cars before individual owners’ cars — and that they could face lawsuits from rental companies for lost revenue while recalled cars are out of commission.

The alliance represents Detroit’s Big Three automakers, Toyota, Volkswagen and seven others — alongside the National Automobile Dealers Association.

National Highway Traffic Safety Administration chief David Strickland said the bill would fix "a very serious gap in federal law", adding that major rental car firms repair just 50% of recalled vehicles in the first four months.

The legislation also is endorsed by the American Car Rental Association, Consumers for Auto Reliability and Safety, AAA, Advocates for Highway and Auto Safety, Consumers Union, and State Farm Insurance.

The senate bill would require rental cars under a safety recall to be taken off the road as soon as possible but no later than 24 hours after receiving a safety recall notice.

New car dealers are prohibited from selling a recalled automobile but rental car companies are not barred from renting or selling one. Under the new legislation, NHTSA would for the first time, have authority to investigate rental car companies’ recall safety practices.

Rental cars make up 1% of all US vehicles. Both Avis and Enterprise noted that taxis, limousines and car services aren’t covered by the legislation.