The agency also added a new engine -- the 3.0-liter diesel six-cylinder -- and several more models to the list of vehicles it says were illegally equipped. The models include the 2014 VW Touareg, the 2015 Porsche Cayenne and the 2016 Audi A6 Quattro, A7 Quattro, A8, A8L and Q5.
The 3.0-liter vehicles spew emissions of nitrogen oxide up to nine times the EPA standard, the agency said.
The Clean Air Act violation notice issued Monday covers about 10,000 diesel passenger cars sold in the U.S. since model year 2014. It also includes an unknown number of 2016 vehicles.
"VW has once again failed its obligation to comply with the law to protect clean air for all America," said Cynthia Giles, assistant administrator for the EPA's Office for Enforcement and Compliance Assurance.
Giles said the EPA had "clear evidence" of the violations and that an investigation was ongoing.
In September, the automaker admitted to using illegal software "defeat devices" to cheat U.S. pollution tests in four-cylinder diesel vehicles after being confronted by federal and state regulators.
The scandal led to the resignation of company Chief Executive Martin Winterkorn and multiple federal investigations, and could result in up to $18 billion in federal fines.
VW replaced Winterkorn with Matthias Mueller, 62, the top manager of its Porsche subsidiary, making the implication of the Porsche Cayenne all the more notable.
At the time of his appointment, Mueller said his “most urgent task” was to win back trust for VW by “leaving no stone unturned and with maximum transparency.”
Mueller had headed Porsche since 2010.
Giles said the EPA and the California Air Resources Board discovered the "defeat devices" on 3.0-liter vehicles through the testing program. She said she could not comment further on any discussions with VW and refused several times to answer questions about whether the automaker lied about these cars and others.
Giles said VW designed the software to have the 3.0-liter engine run on two modes: “temperature conditioning” – the cheating mode – and “normal mode.”
The car reverted to normal mode “exactly one second” after the software detected that EPA testing conditions had stopped, Giles said.
VW has blamed a few rogue engineers for the scheme.
The list of affected vehicles could still grow as California and federal environmental authorities continue their investigation. The testing will include diesels from other manufacturers, such as BMW and Mercedes-Benz, which have not been implicated.
"Today, we’re requiring VW Group to address these issues," said Richard Corey, executive officer of the California Air Resources Board. "This is a very serious public health matter. We're continuing to conduct a rigorous investigation of more vehicles until all the facts are out in the open."
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