How VW software cheat figured out when EPA test was being run — and how VW got busted

It was a group of West Virginia academics and students working for the Center for Alternative Fuels, Engines, and Emissions (CAFEE), contacted by the International Council on Clean Transportation to conduct real-world tests on light-duty diesel vehicles who first smelled a rat when they took several cars on a road trip to evaluate the tailpipe emissions of diesel cars manufactured for the U.S. market by European auto makers, something on which no published academic research existed. They then alerted the EPA and the California Air Resources Board (CARB), who then did what government agencies should: due diligence. From Pete Bigelow at Autoblog:

     The road trip was Volkswagen's undoing. When the West Virginia team returned to Los Angeles, they were befuddled by the test results. In theory, the Passat should have spewed the lowest levels of pollutants among the three cars. Equipped with the more modern selective catalytic reduction technology, the team expected to find minimal levels of nitrogen oxide. But the car, which had been certified at a California Air Resources Board facility prior to the start of the road trip, had elevated levels of NOx that were 20 times the baseline levels established beforehand.
      The researchers, comprised of professors Gregory Thompson and Dan Carder and students Marc Besch and Thiruvengadam, knew their on-board equipment functioned properly because, early in their research, they had double-checked its accuracy after recording sky-high NOx readings from the Jetta that showed 30 times the level of its baseline testing at the CARB facility. It was particularly noteworthy because the Jetta contained the first-generation Lean NOx Trap technology, not the more efficient SCR, yet both produced large discrepancies. The BMW, on the other hand, performed as expected.

And how did the software know the vehicle was subjected to an EPA test so it could turn on the vehicle's emissions control equipment just for the test? ComputerWorld described how easy that would be:

     ...modern cars can sense when a hood is open for dynamometer testing, "so a smart hood switch could double as a defeat device."
     Or, another sensor could detect when a vehicle's traction control unit was disabled, which is required during emissions testing, and place the emission system into a different mode.
     "The possibilities are almost endless," [Arvind Thiruvengadam] told Autoblog. "I'm pretty sure that if you're one of the largest car manufacturers, you could do a lot more."
     According to the EPA's violation letter to Volkswagen, dated Sept. 18, "the position of the steering wheel, vehicle speed, the duration of the engine’s operation, and barometric pressure" -- all very specific indicators of an emissions test -- acted as the activation switch for the "defeat device".  

Below, a 1997 VW ad for its Golf, aired during the coming out episode of Ellen. VW denied that the actors in the ad were depicting a gay couple and that settled any such suspicions because HEY! — would Volkswagen lie to you?