In its letter sent to vehicle manufacturers, U.S. EPA said it is adding evaluations designed to look for potential defeat devices to its confirmatory testing. This comes after EPA sent Notice of Violation of the Clean Air Act to Volkswagen AG, Audi AG, and Volkswagen Group of America, Inc. alleging that 4-cylinder Volkswagen and Audi diesel cars (model years 2009–2015) include software, known as a "defeat device," that circumvents EPA emissions standards.