Employees of Japanese car-maker Mitsubishi have been faking fuel consumption data since 1991, an internal investigation has revealed.
Mitsubishi launched the investigation last week after its partner Nissan pointed to inconsistencies in fuel data for its Dayz Roox vehicles, which are manufactured for Nissan by Mitsubishi.
More than 600,000 vehicles have been affected including Mitsubishi’s own eK Wagon and eK Space models.
"We don't know the whole picture and we are in the process of trying to determine that," said Mitsubishi’s president Tetsuro Aikawa, adding that the probe has not been concluded yet and more irregularities might be revealed in the process.
Aikawa, who previously accepted responsibility for the wrongdoings, said he was not aware of the practices and does not understand the motivation.
All of the affected vehicles fall into the ‘minicars’ category for which fuel economy is the major attraction. Most of the cars have been produced after March 2013 when a mileage goal previously set in 2011 was suddenly raised.
Mileage fraud is a violation of Japan's fuel efficiency law for cars because buyers are eligible for tax breaks if a vehicle model delivers good mileage.
Aikawa said the firm would only decide about compensations for customers after gaining a better understanding of the full extent of the fraud. The firm has suspended sales and production of the affected vehicles for the duration of the investigation.
Mitsubishi released two diagrams explaining how road tests were manipulated. It previously said the engineers tampered with the data by manipulating tyre pressure during testing.
The company, which also makes the Outlander sport-utility vehicle and the i-MiEV electric car, has arranged for a panel of three lawyers, including a former prosecutor, to further investigate the scandal from an outsider's point of view, with a report expected within three months.
Mitsubishi has been struggling to regain public trust since a scandal involving cover-ups of car defects was revealed 15 years ago. The firm had to explain its attempts to conceal serious problems including failing breaks, malfunctioning clutches and fuel tanks prone to falling off. The cover-ups dated back as far as the 1970s. More than a million vehicles had to be recalled retroactively.
A 2002 incident left an additional mark on Mitsubishi’s reputation after a wheel rolled off a Mitsubishi truck and killed a pedestrian.
Japan is periodically shaken by scandals at top-name companies, including electronics company Toshiba, which had doctored accounting books for years, and medical equipment company Olympus, which acknowledged it had covered up massive losses.