Explaining Volkswagen’s Emissions Scandal

The software sensed when the car was being tested and then activated equipment that reduced emissions, United States officials said. But the software turned the equipment down during regular driving, increasing emissions far above legal limits, most likely to save fuel or to improve the car’s torque and acceleration. The software was modified to adjust components such as catalytic converters or valves used to recycle some of the exhaust gasses. The components are meant to reduce emissions of nitrogen oxide, a pollutant that can cause emphysema, bronchitis and other respiratory diseases.

The on-road testing in May 2014 that led the California Air Resources Board to investigate Volkswagen was conducted by researchers at West Virginia University. They tested emissions from two VW models equipped with the 2-liter turbocharged 4-cylinder diesel engine. The researchers found that when tested on the road some cars emitted almost 40 times the permitted levels of nitrogen oxides.

The Environmental Protection Agency said in September that it would order Volkswagen to recall seven of its American car models with affected engines, which amount to nearly 600,000 vehicles. Volkswagen has not released a list of international models, which make up the majority of the 11 million affected vehicles, that have engines with the software in question.

The E.P.A. said on Nov. 2 that it had found the same test-cheating software on additional Volkswagen and Audi diesel models and on a Porsche model. The agency said it covered about 10,000 cars sold in the United States since the 2014 model year. But, in meetings with the E.P.A. on Nov. 19 and 20, the company admitted that all model years since 2009 with its 3-liter diesel engines contained the software as well. The latest disclosure covers an additional 75,000 vehicles.

Volkswagen has set aside 17.8 billion euros, or about $19.9 billion, for costs related to the scandal. But the American settlement with the government and car owners will consume a big chunk of that money. The company has recently recorded losses, and internally it has shaken up its leadership. Its chief executive, Martin Winterkorn, as well as the head of its American operations have stepped down, and the company suspended several high-ranking executives. The company continues to face legal challenges, including civil and criminal investigations in the United States, Germany and other countries. On Sept. 9, an engineer at the company who was was central in the development of the software pleaded guilty to criminal charges related to the scandal. One big risk to the carmaker is in Europe. The United States deal focuses on nearly 500,000 Volkswagen vehicles, but the carmaker admitted to installing the cheating device on more than 11 million cars worldwide, with 8.5 million in Europe.