According to the Justice Department plea deal, Liang and his co-conspirators were tasked with developing the "EA 189" diesel engine for sale in the US. However, they quickly realized that they could not design an engine that could meet the stringent US emissions limits. So instead, they designed and implemented software that would recognize when the car was being smog tested -- versus simply driving around normally -- and adjust the vehicle's emissions accordingly.

Between 2009 and 2016, Liang and his co-conspirators reportedly repeatedly lied about the capabilities of the engines as well as their ability to meet the Clean Air Act statute and concealed the existence of the illicit software from US regulators. The terms of his plea deal have not yet been made public.