The EPA has accused Volkswagen AG, Audi AG and Volkswagen Group of America, Inc., of using defeat devices, which is software installed in vehicles that enacts emissions controls only when their vehicles are undergoing emissions testing. When the cars are driving normally, their emissions could be up to 40 times the NOx emissions standards, which are set by the EPA in the interest of public health.
The cars containing the defeat device software are four-cylinder Volkswagen and Audi diesel cars from 2009 to 2015 model years.
The EPA’s latest letter to the manufacturers informs them that additional testing will be carried out in the future to search for such devices.
EPA Testimony Statements -- http://www.epa.gov/ocir/hearings/testimony/113_2013_2014/2013_2.htm