Nearly two-thirds of the U.S. vehicles containing defective air bag inflators made by Takata Corp. remain unrepaired as automakers have made varying degrees of progress addressing the largest auto recall in U.S. history.

As of mid-September, 20 million vehicles containing defective Takata air bag inflators still haven’t been fixed, 64 percent of the 31.5 million vehicles containing the defective parts, according to a progress report released Friday by John Buretta, the independent monitor overseeing the recalls and a former U.S. prosecutor.

In total, 43 million inflators have been recalled to date and 25 million still need replacements, as some vehicles have two defective air bags. The recalls are set to swell to about 65 million inflators by the end of 2018 under a National Highway Traffic Safety Administration plan to replace the parts in phases, scheduling the riskiest parts for repair first.

The defective inflators can explode in a crash, showering vehicle occupants with metal shards. The parts have been linked to 13 deaths in the U.S. and hundreds of injuries. Mounting liabilities from the recalls pushed Takata to file for bankruptcy in June.

The report concluded that the auto industry as a whole is beginning to make “meaningful progress” developing effective strategies for the recalls while noting “there remains much room for improvement.” It did not identify which automakers were the leaders or laggards.

According to the report, repair rates for the 19 automakers affected by the recalls “vary widely,” reflecting “uneven historical efforts” to alert customers and take other steps to complete the campaigns.

“Recalling these inflators requires a substantial dedication of resources and planning by vehicle manufacturers to ensure that recall efforts remain effective on a national scale,” the report said.

Motorists can check the status of their vehicles at this site: https://www.nhtsa.gov/recalls.