Volkswagen is said to have installed sophisticated software known as "defeat devices" that only turned on full emissions controls when it sensed official testing taking place, but otherwise emitted 10 to 40 times the legal amount while on the road. The issue was first brought to light by the Environmental Protection Agency earlier this month, based on tests from West Virginia University.
Volkswagen estimates 11 million cars are affected worldwide. Mueller contended that Volkswagen vehicles never posed safety risks to consumers.
Winterkorn, who resigned as Volkswagen's CEO after a board meeting Wednesday, said he was shocked by the events and "stunned that misconduct on such a scale was possible in the Volkswagen Group," according to a company statement.
Mueller's appointment ushers in a new leadership pairing, with former chief financial officer Hans Dieter Poetsch set tobecome chairman in November. Volkswagen is still searching for his replacement.
Former chairman Ferdinand Piech resigned in April after losing a leadership battle with Winterkorn.