Details have surfaced on exactly what Volkswagen did to trick the EPA’s emissions test.
The Volkswagen and Audi diesel vehicles in question feature a 'Dyno' and 'On Road' mode, the previous of which is used for testing purposes when a vehicle is on a dynamometer. Since testing for vehicles is often done on dynos, a test mode is necessary so that other safety features such as traction control and stability control do not kick in. While having a testing mode or dyno mode isn’t illegal or even misleading, Volkswagen’s version had its diesel engines perform differently than they would in the real world.
Essentially, when the vehicles were put into the dyno mode for testing, the engines were detuned to meet emissions targets. Once the vehicle was restarted and taken out of test mode, they emitted 40 times the federal standard of nitrogen oxide.
Currently, the affected models from VW’s diesel scandal include: 2009-2015 Audi A3, 2009-2015 Volkswagen Jetta and Jetta SportWagen, 2009-2015 Volkswagen Beetle and Beetle Convertible, 2009-2015 Volkswagen Golf, 2014-2015 Volkswagen Passat and 2015 Volkswagen Golf SportWagen.