The ongoing controversy around Volkswagen's emission scandal has shocked the officials at the United States Environmental Protective Agency (U.S. EPA) with its CEO, Martin Winterkorn announcing his retirement on Wednesday, September 23. The German automaker is facing accusations of cheating because of its misleading advertisements around diesel cars promising better mileage and cleaner emissions. The Volkswagen power plant is seen from the railway station in Wolfsburg, Germany Here is all you need to know about the Volkswagen emissions scandal:

1. It started when the U.S. EPA and CARB served notice to Volkswagen Group that approximately 480,000 VW and Audi automobiles equipped with 2-litre TDI engines, and sold in the US between 2009 and 2015, had an emissions-compliance "defeat device" installed.

A diesel Volkswagen Passat TDI SEL is taken away by a tow truck for having an expired registration, in Santa Monica, California, United States

2. This device detects emissions testing conditions and makes the automobile comply with it with the help of a software.

But under normal driving conditions, it suppresses the emission controls thus producing more torque for getting better fuel economy. And all this was done at the cost of emitting nitrogen oxides 40 times more than the prescribed limit by the law. The affected car models were Beetle, Passat, Jetta, Audi A3 and Golf.

The exhaust system of a Volkswagen Passat TDI diesel car is seen in Esquibien, France