U.S. and California environmental regulators said Friday they are investigating whether Volkswagen deliberately circumvented clean air rules on almost 500,000 diesel cars.

The Environmental Protection Agency (EPA) said Volkswagen allegedly used software in four-cylinder Volkswagen and Audi diesel cars from model years 2009 to 2015 to circumvent emissions testing of certain air pollutants.

“Put simply, these cars contained software that turns off emissions controls when driving normally and turns them on when the car is undergoing an emissions test,” Cynthia Giles, an enforcement officer at the EPA, told reporters in a teleconference.

The feature, known as a defeat device, results in the cars emitting as much as 40 times the legally permissible emissions, Giles said.

“Using a defeat device in cars to evade clean air standards is illegal and a threat to public health,” Giles said.

Volkswagen sold roughly 482,000 diesel passenger cars in the United States since 2008. The vehicle models include the Jetta, Beetle, Golf, Passat and Audi A3.

The EPA called on the German automaker to fix the cars’ emissions systems, but said car owners do not need to take any immediate action.

Volkswagen said in a statement it is cooperating with the investigation.

Last year, Honda agreed to spend $267 million, and Ford, $7.8 million, in settlements with the EPA over “defeat devices” in vehicles from the 1990s.

Al Jazeera and wire services