Federal officials are stepping up their investigation into complaints about stalling Corolla and Matrix vehicles manufactured by Toyota Motor (TM).
In a statement, the National Highway Traffic Safety Administration said it's moving beyond a preliminary evaluation of the problem to more in-depth scrutiny to examine engine problems such as stalling, harsh gear shifting or inability to start.
The investigation into 2005, 2006 and 2007 model year Corolla and Corolla Matrix cars in the U.S. began Aug. 18, NHTSA said.
Toyota spokeswoman Monika Saito in Tokyo confirmed the NHTSA investigation, and said Toyota was cooperating fully but declined to give more details, the Associated Press reported.
The problems may be caused by faulty engine control units, an issue that Toyota is assessing, NHTSA said. Whether the cars will be recalled remains unknown.
Toyota has battled recall woes nearly all year. Executives came under fire in February during congressional testimony looking into unintended acceleration problems in Toyota and Lexus brand vehicles. Toyota has said the problem is caused by either sticky gas pedals or rubber floor mats that jam the accelerator to the floor. It has recalled about 8 million vehicles worldwide to address those problems.
The world's largest automaker has repeatedly denied electronics as a source of unintended acceleration. Still, NHTSA has asked NASA and the National Academy of Sciences to look into whether Toyota cars' electronics could be defective. Earlier this month, regulators told Congress that data recorders pulled from crashed vehicles exhibiting the problem didn't reveal any new safety defects.
Toyota has also recalled thousands more vehicles for a variety of problems, including braking and faulty software.
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