The European Court of Justice will advise whether to impose heavy penalties over vehicle pollution.
Speaking to German newspaper Die Tageszeitung, which uncovered the error, Dieter Köhler said: “I practically do everything by myself and, as a pensioner, I don't even have a secretary anymore.”
Three years on from dieselgate Europe considers the future of diesel.
A Berlin court ruled on Tuesday that the German capital should impose driving bans on some roads for diesel cars whose emissions of nitrogen oxide exceed permitted limits.
New emissions standards have led car makers to stop selling many of their most powerful models.
BMW Group and Daimler claim they were not aware the exhaust fumes tests were being tested on humans and monkeys — while Volkswagen concedes some of its employees may have been aware of the testing at the time.