The US government's Environmental Protection Agency issued the company a notice of violation this afternoon, accusing Volkswagen of having installing a "defeat device" into its four-cylinder Volkswagen and Audi vehicles between 2009 and 2015, according to the New York Times.
The device is programmed to detect when the car is undergoing emissions test, during which time it will turn on its full emissions control systems. These controls are reportedly then turned off when the car is driving in "normal" situations, when the cars pollute more heavily than reported by Volkswagen, the EPA claims.
The government agency estimates that 482,000 diesel passenger vehicles are affected.
Volkswagen could not be contacted at the time of publication.