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Takata, 11 automakers that used its air bag technology and U.S. safety regulators are pursuing separate efforts to determine the root cause of problems linked to at least six deaths.
A subcommittee of the House Energy and Commerce Committee has scheduled a hearing on June 2 on the problems that led to the recall.
Replacement inflators that are currently being installed could eventually need to be replaced if it turns out that the real problem was not addressed before Takata began making parts to fix about 34 million vehicles covered by the expanded U.S. recall announced last week, several industry officials familiar with the probes said. The air bags can explode with too much force, causing shrapnel to fly out and hit drivers and passengers.
"If you don't find out the root cause, who knows? We may have this same discussion again in four, five, six, seven, 10 years," said David Kelly, a former acting NHTSA administrator now charged with leading a consortium of 10 automakers investigating the Takata inflators.
Kelly did not say how soon his group would finish its work. "It is apparent to us that we have a lot of work in front of us," he said.
"We are confident that our new air bags are safe," Takata said in a statement on Tuesday. The company has organised an "independent Quality Assurance Panel" led by former U.S. Transportation Secretary Samuel Skinner. That panel is conducting "a comprehensive review to ensure Takata's current manufacturing procedures meet best practices," the company said.
The Japanese supplier has been investigating air bag problems for more than seven years, but says it has narrowed the search only to "preliminary conclusions" linked to multiple factors.
Takata has said inflators exposed over several years to high humidity combined with cycling between extreme high and low temperatures are at risk. Too much moisture can cause the ammonium nitrate in the air bag propellant to break down, increasing the risks of a violent explosion, chemical experts and analysts have said.
Takata cited other factors, including the design of the inflator or the air bag, which may be prone to leaky tape seals in some cases; the shape of the explosive propellant used to deploy the air bag, and vehicle design. In past recalls, Takata has blamed manufacturing mistakes and problems with proper storage of propellant.
"It's a confluence of a whole series of issues that have been going on for a long time," said Sean Kane, president of Safety Research & Strategies, a consultant with plaintiffs' attorneys, government organizations and companies.
INFLATOR PROPELLANT UNDER MICROSCOPE
Takata said it has reviewed more than 45,000 inflators over the past year in ballistic tests, live dissections, propellant analysis for moisture, chemical analysis, air and helium leak tests and CT scans.
Meanwhile, automakers that used Takata air bag hardware have stepped up efforts to determine the cause of the explosions. Honda Motor Co, which built the vehicles involved in all six of the fatalities attributed to Takata air bags, last autumn bought used and scrapped cars in Japan to conduct its own tests. Two senior Honda insiders said then that the testing indicated shortcomings in Takata's manufacturing quality.
Federal safety regulators recently hired research group Battelle to probe for the cause - a task made more complex because automakers used four different types of inflators across a wide range of models.
"We have a lot of work to do, especially with regard to why this happened in the first place," U.S. Transportation Secretary Anthony Foxx said last week.
National Highway Traffic Safety Administration head Mark Rosekind said last week replacement inflators "are safer. The concern is, are they safe over the long term? That has yet to be determined."
The explosive propellant Takata still uses to power its inflators - ammonium nitrate - is not in the clear, said officials familiar with the various probes. The material is less expensive and cleaner burning than other options, but it can be highly volatile, said industry officials and chemists.
To make the propellant mix safer, Takata more recently has added desiccant, a material that gathers up and holds moisture chemically and should extend the lifetime of the inflator by two to three times, said a person familiar with the change.
