BERLIN (AP) — German automaker Opel reportedly faces questions over emissions from its diesel vehicles, an issue that's already tarnished many of its competitors.

Germany's transport ministry confirmed a report by weekly Bild am Sonntag late Friday that authorities have asked the company to answer questions about the way the emissions cleaning system in three of its models works.

Bild am Sonntag reported investigators have found evidence that the cars switch off their emissions cleaning system while driving, for no apparent reason.

Opel's rival Volkswagen was found three years ago to have used a so-called defeat device to ensure its diesel cars reduce emissions in official tests, but not during regular driving.

VW has paid hefty fines and remains under criminal investigation.

French automaker PSA bought Opel from General Motors last year.