Researchers first raised concerns about Volkswagen diesel emissions more than a year ago, culminating in the disclosure of alleged "defeat devices" installed in 482,000 Volkswagen diesel cars from the 2009-2015 model years. Below is a timeline of major events; we'll update this as the story unfolds.
Related: Volkswagen Diesel Recall: What We Know
According to letters sent to VW on Sept. 18 from the Environmental Protection Agency and the California Air Resources Board, here's a deeper dive on what happened. When the EPA conducts emissions testing, automakers have to disclose "auxiliary emissions control devices," according to the EPA's letter. These AECDs affect a car's emissions system based on situational factors, such as how hard the engine is running or how hot it is. The EPA calls AECDs "defeat devices" only if they reduce the effectiveness of emissions controls outside of testing parameters and are not necessary for safety or engine startup.
May 2014: Researchers at West Virginia University and the International Council on Clean Transportation publish findings that find "significantly higher in-use emissions" in a 2012 Jetta TDI and 2013 Passat TDI, according to the EPA. (Volkswagen markets its diesel cars as TDI.) Volkswagen tells regulators that the differences amount to technical issues and "unexpected" test conditions.
December 2014: Volkswagen agrees to voluntarily recall its diesel cars to address the emissions issues.
May 2015: CARB tests the updated emissions on a 2012 Passat TDI, in a lab and on the road. The agency finds some improvement, but not enough.
July: On July 8, CARB shares its findings with VW. None of the technical issues suggested by the automaker are found to explain CARB's results. Between July 8 and Sept. 3, CARB and the EPA say they will not certify VW's 2016 diesel lineup, a necessary step to put those cars on sale. Only then does VW admit to software irregularities. The automaker discloses that the software calibrations in three separate diesel emissions systems had "a second calibration intended to run only during certification testing," according to CARB.
Sept. 3: VW admits that the cars were "designed and manufactured with a defeat device to bypass, defeat or render inoperative elements of the vehicle's emission control system," CARB says.
The software uses "dyno" and "road" calibrations that read when an emissions test is being conducted; when the car is not being tested, the "road" calibration dials back the effectiveness of two types of emissions-treatment systems: nitrogen oxide traps and selective catalytic reduction (a urea solution). When dialed back, the systems allow the engine to emit nitrogen oxide levels that are 10 to 40 times the allowable amount by the EPA.
Sept. 18: The EPA concludes that such software constitutes an AECD defeat device, and says that VW has violated the Clean Air Act. The cars in question should not have been EPA-certified, and the Department of Justice can enforce up to $37,500 in civil fines per vehicle. In total, that means VW could face fines ranging up to $18 billion.
Sept. 21: VW confirms that it has ordered dealers to stop the sales of all four-cylinder diesel cars, as well as the four-cylinder diesel Audi A3. (Audi is a luxury brand in the Volkswagen Group.) The automaker also confirms a stop-sale order for certified pre-owned cars with the four-cylinder diesel.
Sept. 22: VW announces that some 11 million diesel cars worldwide have the same "defeat device" software that evades emissions testing. The automaker says it's set aside 6.5 billion euros, or $7.3 billion, to cover the cost of fixing affected cars.
Sept. 23: Volkswagen CEO Martin Winterkorn resigns, saying in a statement that he's unaware of any personal wrongdoing but accepts responsibility for the crisis. Winterkorn says his resignation clears the way for a "fresh start" at Volkswagen.
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