Revelations that engineers for German car giant Volkswagen and its subsidiaries used technology to cheat emission regulations shook the industry. Five years on, those at the helm during the scandal are standing trial.
Stadler, 57, appeared before the Munich district court to answer charges of fraud, falsifying certifications and false advertising.
Former Audi and Porsche manager Wolfgang Hatz and two Audi engineers are being tried alongside Stadler, facing charges of fraud.
German car giant Volkswagen — whose subsidiaries include Porsche, Audi, Skoda and Seat — admitted in September 2015 that it had installed software to rig emissions in 11 million diesel vehicles worldwide.
Criminal proceedings have taken place in the United States over the scandal, leading to the imprisonment of two Volkswagen employees. In Germany, there have been no convictions over the scam.
Prosecutors opened the proceedings on Wednesday by reading aloud an over 90-page indictment.
The trial is expected to last more than two years. If found guilty, the defendants face up to 10 years in jail.
Volkswagen has always insisted that the scam was the work of a handful of lower-level employees acting without the knowledge of their superiors, but prosecutors dispute this.
They said Stadler knew about the scam by the end of September 2015 "at the latest" but nevertheless allowed thousands more vehicles fitted with illegal defeat devices to be sold.
According to the public prosecutor's office, an engine developer identified as Giovanni P. has largely confessed to the charges, while his former colleague Henning L has fully confessed.
"He was not a decision-maker and he was bound by instructions. He received instructions," Walter Lechner, a defense lawyer representing Giovanni P. said before entering the trial venue on Wednesday, promising that "many, many details" about the scandal would be aired.
Stadler's legal representation did not make any comment before heading into the venue.