Several cars made by Volkswagen and its subsidiaries were found to come with software that let them get around emission standards by deliberately lowering emissions when the cars are subject to emission norm tests.
The cars are not facing recall at this time, the EPA said. California, the EPA and the Justice Department are working together on an investigation of the allegations.
The effectiveness of these vehicles' pollution emissions control devices is greatly reduced during all normal driving situations.
Prior to issuing the violation notice, the EPA said it worked with the California Air Resources Board and would continue to investigate "these very serious matters". The device is programmed to detect when the vehicle is undergoing official emissions testing, and to only turn on full emissions control systems during that testing.
Exposure to nitrous oxide has been linked to various health issues including increased asthma attacks and other respiratory illnesses. It is likely that owners will receive a recall notice from Volkswagen in the future, it said.
"Our goal now is to ensure that the affected cars are brought into compliance, said Richard Corey, the executive officer for California's Air Resources Board". "By making and selling vehicles with defeat devices that allowed for higher levels of air emissions than were certified to EPA, Volkswagen violated two important provisions of the Clean Air Act".
Volkswagen admitted that it had installed the defeat device when questioned by investigators.
"VW is cooperating with the investigation", said VW spokeswoman Jeannine Ginivan.
The alleged violations cover approximately 482,000 cars sold in the United States since 2008, including the Jetta, Beetle, Audi A3 and Golf - all with model years 2009 -15 - as well as the Passat for model years 2014-15. We have done multiple comparisons of diesel and gasoline powered cars, and also comparisons of diesel and hybrid cars, which came to the opposite conclusion.