A House of Representatives committee sought more information from Toyota Motor Corp on Friday about the automaker's review of electronic throttles.
The Energy and Commerce Committee also scheduled a May 6 hearing focusing on work for Toyota by Exponent Inc and asked that the Japanese automaker's U.S. sales chief, Jim Lentz, testify.
The development came ahead of Monday's deadline for Toyota to decide whether to appeal a proposed $16.4 million Transportation Department fine for allegedly delaying a January recall for gas pedals that would not spring back as designed.
Regulators concluded that Toyota knowingly hid the defect for months from the government. Toyota has said it had done a poor job of communicating before its recalls, and has taken steps to address the problem.
Exponent was hired by the automaker to examine throttle systems that are at the center of questions about unintended acceleration in Lexus and Toyota vehicles.
Preliminary work by Exponent showed no problems with the throttles, Toyota has said. The automaker has also said its internal reviews over the years have found that the system is sound.
Toyota launched recalls of more than 6 million cars and trucks across its product line in October 2009 and in January for unintended acceleration linked to the sticky pedal problem and for floor mats that can trap the accelerator.
U.S. safety investigators have never found any problems with the software-driven Toyota throttle systems, but lawmakers, safety advocates and others have questioned whether investigations over the years were thorough enough.
National Highway Traffic Safety Administration (NHTSA) engineers are in the midst of another review to address lingering questions about whether there are possible glitches in Toyota throttles and whether they could be behind complaints of unintended acceleration.
In a letters to Lentz and Exponent officials, Energy and Commerce Chairman Henry Waxman and Investigations Subcommittee Chairman Bart Stupak sought contracts, correspondence, reports and analysis related to Exponents work for Toyota.
The Democrats set an April 26 deadline for the companies to respond.
Toyota said in a statement that it is more than willing to meet with the committee and discuss electronic throttle testing and steps to improve quality assurance.
Waxman and Stupak took issue with Toyota's conclusion in March that it had rigorously examined its throttle systems and found no problems.
Toyota challenged a key committee witness who testified at a hearing in February that he found a flaw during his own testing of circuitry in a Toyota Avalon.
Toyota has already turned over tens of thousands of documents to congressional committees and regulators examining its response to unintended acceleration, the recalls and the throttle review.
(Reporting by John Crawley; Editing by Andre Grenon and Richard Chang)