“That’s wrong,” Mr. Rouen, 63, said. “That is just wrong.”

Drivers like Mr. Rouen report monthslong delays in getting their vehicles serviced at dealerships, for lack of replacement parts. Just under 30 percent of recalled inflaters had been fixed as of Jan. 15, according to the safety agency. That was before regulators added five million more inflaters to the 23 million already under recall.

And it may be years before regulators have any firm understanding of the scope of the problem. The safety agency, which has barred Takata from using ammonium nitrate for new orders, has given the supplier until the end of 2018 to prove that ammonium nitrate is safe in existing airbags. And Takata has even longer, until the end of 2019, to show that inflaters with a more advanced version of the compound are safe.

“Frankly, everything’s on the table with respect to any Takata inflater that uses ammonium nitrate,” said Sean Kane, founder and president of Safety Research & Strategies, an auto safety consultancy, who is involved in litigation against Takata.

Image The metal fragment from a Takata airbag that killed Joel Knight.

“It takes another death or an injury to get additional vehicles recalled,” he said. “Unfortunately, Joel Knight paid for that with his life.”

In Mr. Rouen’s case, he has been waiting for a repair since he received a recall notice last July, and he drives the car as little as possible. He filed a complaint with the South Carolina Department of Consumer Affairs. In August, Ford responded to the agency, explaining that the car had been deemed safe to drive until parts were available.

“As such, we will not be meeting Mr. Rouen’s request for a rental vehicle until the repairs are completed,” the letter said.