Photo of 2015 Golf TDI engine courtesy of VW.

Photo of 2015 Golf TDI engine courtesy of VW.

Audi and Volkswagen four-cylinder diesels used software to circumvent federal emissions standards, the U.S. Environmental Protection Agency has alleged.

The cars from the 2009 to 2015 model years included a "defeat device" — a sophisticated software algorithm that detects when the car is undergoing official emissions testing and turns on full emissions controls during that test, according to an EPA violation notice issued Sept. 18. This would violate the Clean Air Act.

The allegations cover about 482,000 models, including the Jetta TDI, Beetle TDI, Golf TDI, and Audi A3 TDI. The Passat TDI is affected from the 2014-MY and 2015-MY.

“Using a defeat device in cars to evade clean air standards is illegal and a threat to public health,” said Cynthia Giles, assistant administrator for the EPA's Office of Enforcement and Compliance Assurance.

In a separate action, the California Clean Air Resources Board has issued an In-Use Compliance letter. CARB has initiated its own investigation in addition to the EPA probe.

"Our goal now is to ensure that the affected cars are brought into compliance, to dig more deeply into the extent and implications of Volkswagen's efforts to cheat on clean air rules, and to take appropriate further action," said Richard Corey, CARB executive officer.

By using the defeat device, the cars will emit nitrogen oxides (NOx) at up to 40 times the regulatory standard during normal operating conditions, according to the EPA.

Volkswagen must now offer a fix for affected models, which remain legal to drive and resell. Volkswagen had not commented publicly on Sept. 18.

EPA and CARB uncovered the defeat device software after independent analysis by researchers at West Virginia University, working with the International Council on Clean Transportation, a non-governmental organization, raised questions about emissions levels. The agencies began further investigations into the issue. In September, after EPA and CARB demanded an explanation for the identified emission problems, Volkswagen admitted that the cars contained defeat devices, according to EPA.

Read the EPA's full release here.