According to the Environmental Protection Agency (EPA), VW cars with bigger diesel engines also contained software devices designed to cheat in emissions tests.
Porsche, Audi and VW cars are all included in this new investigation, which affects at least 10,000 vehicles.
The EPA said that cars with 3.0 liter engines from the years 2014 to 2016 were affected.
However, VW denies the vehicles have software designed to cheat tests.
Instead the automaker says that cars with the 3.0 liter diesel V6 engines “had a software function which had not been adequately described in the application process”.
VW said it was cooperating with the EPA to “clarify the matter”.
“Volkswagen AG wishes to emphasize that no software has been installed in the 3-liter V6 diesel power units to alter emissions characteristics in a forbidden manner,” the company said in a statement.
Meanwhile, Porsche said it was “surprised” by the EPA’s allegations.
“Until this notice, all of our information was that the Porsche Cayenne diesel is fully compliant,” it said in a statement.
The EPA says the investigation is ongoing.
“VW has once again failed its obligation to comply with the law that protects clean air for all Americans,” said Cynthia Giles, assistant administrator at the EPA’s enforcement unit.
The EPA identified these diesel models as containing software aimed at cheating tests: 2014 VW Touareg; 2015 Porsche Cayenne; 2016 Audi A6 Quattro, A7 Quattro, A8, A8L and Q5.
In early September, VW admitted to the EPA that cars from the model years 2009 to 2015 contained software designed to cheat emissions tests.
It said that 11 million cars were affected.
That prompted US regulators to run further tests designed to detect such defeat devices and led to today’s announcement from the EPA.
“These tests have raised serious concerns about the presence of defeat devices on additional VW, Audi and Porsche vehicles. Today we are requiring VW Group to address these issues. This is a very serious public health matter,” Cynthia Giles said.
Regulators all over the world are now looking at VW’s diesel cars and the company is also facing criminal investigations.
State and federal prosecutors in the US have announced criminal investigations and German prosecutors are looking into the scandal.
VW CEO Martin Winterkorn resigned in late September as the scale of the scandal emerged.
At the time Martin Winterkorn said he was “not aware of any wrongdoing on my part” but was acting in the interest of the company.
Last month Volkswagen reported its first quarterly loss for at least 15 years after taking a big charge to cover the costs of the scandal.
VW said it had set aside €6.7 billion ($7.4 billion) to cover costs related to emissions cheating, which left it with a €2.52 billion pre-tax loss for the third quarter of the year.
Many analysts expect that VW will have to set aside more money to cover the recall of cars, penalties and lawsuits.