According to the EPA, eight models fitted with the company's 3.0-litre V6 turbo-diesel were found to have software that allows it to detect when it is being "tested for compliance with EPA emissions standards" and operate in a "in a low NOx 'temperature conditioning' mode".
The EPA continues: "At exactly one second after the completion of the initial phases of the standard test procedure, the vehicle immediately changes a number of operating parameters that increase NOx emissions and indicates in the software that it is transitioning to 'normal mode', where emissions of NOx increase up to nine times the EPA standard, depending on the vehicle and type of driving conditions. In other tests where the vehicle does not experience driving conditions similar to the start of the federal test procedure, the emissions are higher from the start, consistent with 'normal mode'."
Under normal driving, these vehicles emit up to nine times more oxides of nitrogen (NOx) than is permitted under US federal regulations.
Note that model years for American vehicles typically start being applied in the preceding calendar year.
The federal environmental regulator believes that latest notice of violation covers around 10,000 cars sold from the 2014 model year onward and "an unknown volume of 2016 vehicles".
As with the vehicles equipped with the EA189 turbo-diesel engine, the EPA points out that the latest violation doesn't present an immediate "safety hazard for car owners and drivers and the vehicles remain legal to drive and resell".
As outlined by the EPA, "NOx pollution contributes to harmful ground-level ozone and fine particulate matter" and exposure can lead to asthma attacks, respiratory illness, cardiovascular problems and premature death, especially in the young and the elderly.
This second notice of violation from the EPA comes after Volkswagen admitted to federal and state agencies in the middle of September that it had installed emissions testing defeat devices on its EA189-engined cars in the US. Later it was revealed that up to 11 million cars worldwide, including just under 100,000 vehicles locally, were fitted with this software.
Update (9.12am): Volkswagen AG has just issued a formal denial of the claims asserted by the EPA. The company's full statement reads:
"The United States Environmental Protection Agency (EPA) informed Volkswagen Aktiengesellschaft on Monday that vehicles with V6 TDI engines had a software function which had not been adequately described in the application process. 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.
"Volkswagen will cooperate fully with the EPA clarify this matter in its entirety."