Takata Corp. overlooked internal signs of defective air bags for years and at one point halted global safety audits, according to a congressional report, ratcheting up pressure on the Japanese supplier before a Tuesday Senate hearing.

The report—strongly disputed by Takata—was written by Senate Commerce Committee staff for the panel's ranking Democrat, Sen. Bill Nelson of Florida, and cites employee emails sounding alarms about safety and quality lapses for years before vehicles were recalled.

It also raises questions about whether "an unknown number of replacement parts" for defective air bags are safe, the committee's minority staff said. The report said Takata "should have been aware" of lapses in manufacturing plants as early as 2001, and that the company knew of three serious safety matters tied to faulty air-bag inflaters in the first half of 2007. But a recall wasn't issued until November 2008, the report says.

Problems eventually surfaced in Takata air bags prone to rupturing and spraying shrapnel in vehicles after their propellants degraded over time. The defect has been linked to at least eight deaths and more than 100 injuries world-wide and led auto makers to fix nearly 34 million vehicles in the largest automotive recall in U.S. history.

The 45-page report comes a day before Takata's North American executive vice president, Kevin Kennedy, is set to testify before the full Senate Commerce Committee. He previously faced skeptical House lawmakers alarmed that the company still uses ammonium nitrate in air-bag inflaters, a chemical linked to deadly explosions.

"The report contains a number of inaccuracies based largely on old media articles that Takata has previously refuted, and emails that are taken out of context and characterized in ways that create a false impression," a Takata spokesman said in a statement.

The report also faults federal auto-safety regulators for failing to promptly investigate Takata air bags. The Transportation Department's inspector general, who just released a report criticizing the regulators at the National Highway Traffic Safety Administration, is also set to testify Tuesday, along with the NHTSA chief Mark Rosekind. Mr. Rosekind has said the agency is ready to implement improvement recommendations from the inspector general.

Senate Commerce staffers said they examined about 13,000 Takata emails as part of their report. In one 2011 email exchange cited by the report, a supervisor at the company's Monclova, Mexico, plant refers to improperly welded air-bag inflaters and writes: "We cannot be faced with findings/defects of this sort and NOT do ANYTHING. A part that is not welded=one life less, which shows we are not fulfilling the mission."

In response, a quality engineer says, "We are in a very critical situation because of the most recent problems we have detected on the line. Situations like this can give rise to a Recall."

The report also said Takata stopped global safety audits between 2009 and 2011 for financial reasons. It goes on to detail an internal 2011 audit of the Mexico plant ahead of a company safety director's arrival to conduct an investigation that found "scales with disconnected cables, energetic material on the floor, and dispensers for energetic material on unidentified lines."

The safety director later faulted the plant for not properly closing bags of ammonium nitrate and for storing scrapped or contaminated propellant near good material, allowing for the possibility of a mix-up. That audit also found materials dating back to 2007 were found in an area not meant for long-term storage.

Between February and March 2013, Takata learned of manufacturing problems affecting propellant tablets with certain passenger-side air-bag inflaters at the Mexico plant and another factory in Moses Lake, Wash.

Global audits referenced in the emails "relate to the safe handling by employees of pyrotechnic materials—they were not, as the report implies, related to product quality or safety," the Takata spokesman said. The company "conducts regular reviews of product quality and safety at Moses Lake and Monclova, and at no time were those halted."

Takata and a separate group of 10 auto makers are conducting probes in search of root causes of the air-bag ruptures. Humidity and exposure to moisture have been cited as contributing causes. But some older air bags rupture in such conditions, while others don't, confounding investigators.

"Significant questions remain," the Senate minority report says, citing air bags that perform differently depending on a vehicle's make and model and when it was installed. The report's authors added that some air bags perform properly even under adverse conditions linked to rupturing.

Write to Mike Spector at mike.spector@wsj.com

Access Investor Kit for Takata Corp.

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=JP3457000002

Subscribe to WSJ: http://online.wsj.com?mod=djnwires