Volkswagen Scandal: How a Defeat Device Can Trick Car Emission Tests

Car experts say car technology makes it easy to cheat on emission tests.

"It's totally feasible because cars are so high tech today," Brauer said. "It's fairly easy to make drastic changes to engine tuning in a split second due to the capabilities of computers in cars. It's not only possible but it's easier to do with today's high-tech cars than before."

For emission tests, the tester, such as a dealership or car mechanic, places a sensor in the vehicle's exhaust pipe that measures pollutants while the car is idle and while it is in simulated driving mode during various engine RPMs.

A defeat device, as it is called unofficially, is computer code in a car's electronic control unit that is triggered during emission tests, Brauer said. This software can then temporarily cut emissions, so the engine appears to be much cleaner that it is during normal operation.

The defeat device software programming is forbidden in the U.S. and also in Europe, where diesel cars are much more prevalent. They can come in many forms, but are essentially an algorithm in the electronics or the car's computer system that detects when the car is being tested and switches on full emission controls, explained Christian Stadler, Warwick Business School professor of strategic management, in the U.K.

"But once it is on the road, these controls are switched off," Stadler said.

"As environmental protection and sustainability are among Volkswagen's strategic corporate objectives, the company takes this matter very seriously and is cooperating with the investigation," the company said in a statement on Friday. "Volkswagen is committed to fixing this issue as soon as possible. We want to assure customers and owners of these models that their automobiles are safe to drive, and we are working to develop a remedy that meets emissions standards and satisfies our loyal and valued customers. Owners of these vehicles do not need to take any action at this time."

However, Volkswagen told dealers to halt sales of both new vehicles and certified preowned cars equipped with the four-cylinder 2.0 TDI engine. Audi implemented a stop-sale for its only model with that engine, the A3 2.0 TDI. On Sunday, the company said it launched an external investigation.

Volkswagen CEO Martin Winterkorn said in a statement on Sunday, "I personally am deeply sorry that we have broken the trust of our customers."

But the latest news about Volkswagen is "unfortunate for diesel technology," Brauer said.

"It's already seen by a lot of Americans as dirty compared to traditional gasoline-powered cars, but it isn't," Brauer said. "With modern technology, it's extremely clean."