An American flag flies next to a Volkswagen car dealership in San Diego, California, Sept. 23, 2015. Volkswagen CEO Martin Winterkorn resigned Wednesday, succumbing to pressure for change at the German carmaker, which is reeling from the admission that it deceived U.S. regulators about how much its diesel cars pollute. Reuters/Mike Blake

The Volkswagen emissions scandal, which has resulted in the resignation of CEO Martin Winterkorn and has the automaker facing billions of dollars in penalties and possible criminal charges, has shocked the automotive world. Several models from years 2009-2015 are affected, including the Beetle, the Audi A3 and the Golf.

The automaker used a control mechanism known as a “defeat device” to fool U.S. environmental officials into thinking its cars met emissions standards. But Volkswagen owners shouldn’t go hunting through their cars in search of some mysterious black box. The device exists mostly as additional lines of software code in existing computer controls.

“At this stage, it’s definitely in Volkswagen’s interest to get the issue fixed and behind them as fast as possible,” John German, senior fellow at the International Council on Clean Transportation, told International Business Times. Brown and colleague Peter Mock were the key figures in exposing the scandal.

A defeat device is a type of auxiliary emission control device (AECD) that, in the case of Volkswagen, improved how well the emissions control system worked under certain testing conditions.

The Volkswagen defeat device isn't a physical object: The Environmental Protection Agency explains that the "device" is embedded in the software code responsible for running the engine control computer. The cars in question sensed whether they were being tested based on "the position of the steering wheel, vehicle speed, the duration of the engine's operation, and barometric pressure," according to the EPA.

The control computer is typically stored on the firewall underneath the dashboard.

Smog Alert

When a car is driving down the road, the onboard computer gets data from several different sources. This isn't unusual: this data is used for the daily running of the vehicle. For example, accelerometer data can say how fast it is going, while impact detection can control when to deploy the airbags. Matt De Lorenzo, analyst at Kelley Blue Book, said that the average car has around 20 million lines of code in the onboard computer.

What cars usually do to bring their smog levels down is use an AdBlue or a urea injection. When a small amount of this fluid is injected during the running of the car, it can lower production of NOx, a component of smog. "A lot of larger vehicles use those systems," said Lorenzo. The system used to inject the fluid can be expensive, however, and this can be a concern particularly on the price-sensitive end of the market with the Jettas and Golfs.

So Volkswagen ditched the injectors. Instead, the affected cars’ onboard computers would be measuring and checking the state of the car and, thanks to some extra code, would decide whether the conditions matched those of a lab test. When this happened, the computer would alter the cars’ running states to achieve the efficiency levels required by the EPA.

The EPA claims that cars affected were able to detect when they were under testing conditions. The full emissions controls of the car were only activated during the test. This means that the affected cars were able to trick the test into believing they were more environmentally friendly than they were. “Using a defeat device in cars to evade clean air standards is illegal and a threat to public health,” said Cynthia Giles, assistant administrator for the Office of Enforcement and Compliance Assurance.

Not Quite Lifelike

But how could the car know it was being tested in conditions? Frank O'Donnell, president of Clean Air Watch, explained that the EPA measures cars in laboratories, putting sample cars provided by the manufacturer on a test machine that measures the amount of emissions coming out of the tailpipe.

Volkswagen Group of America Inc. | FindTheCompany

These lab conditions try to simulate real world conditions, but they are still not the same as driving down the road. "It's not like they're going out on the highway with a guy strapped under car, with a device sticking into the tailpipe," O'Donnell said. Telltale signs, like a consistent ambient temperature and the non-drive wheels not turning, could be a giveaway. That means that with the right code the car could "sense" when it is being tested, just by looking for these signs.

It’s not yet known exactly what Volkswagen was measuring. German said that it will probably be months before we find out what exactly the company was doing. The EPA explained that the "device" can't be turned off by the user, and Volkswagen will now be required to fix the issue without the owner incurring charges.