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http://blog.caranddriver.com/everything-you-need-to-know-about-the-vw-diesel-emissions-scandal/

Volkswagen installed emissions software on more than a half-million diesel cars in the U.S.—and roughly 10.5 million more worldwide—that allows them to sense the unique parameters of an emissions drive cycle set by the Environmental Protection Agency. According to the EPA and the California Air Resources Board, who were tipped off by researchers in 2014, these so-called “defeat devices” detect steering, throttle, and other inputs used in the test to switch between two distinct operating modes.

In the test mode, the cars are fully compliant with all federal emissions levels. But when driving normally, the computer switches to a separate mode—significantly changing the fuel pressure, injection timing, exhaust-gas recirculation, and, in models with AdBlue, the amount of urea fluid sprayed into the exhaust. While this mode likely delivers higher mileage and power, it also permits heavier nitrogen-oxide emissions (NOx)—a smog-forming pollutant linked to lung cancer—that are up to 40 times higher than the federal limit. That doesn’t mean every TDI is pumping 40 times as much NOx as it should. Some cars may emit just a few times over the limit, depending on driving style and load.