Volkswagen faces a maximum fine of $18 billion for having diesels that are much dirtier in real world driving than during the EPA’s mandated tests. But how does a car know it is being tested? Consumer Reports explains that the method is simple and uses a feature common in modern cars — but VW is in hot water over how they exploited it.

Buried in the EPA’s letter to VW and explained now by CR is the secret to how the automaker used a hidden mode within the car to achieve the test results they wanted.

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CR, which is more familiar with the actual operations of modern cars than just about any other automotive publication, points out that the diesel VWs in the midst of this regulatory drama have a dyno mode, or a test mode, depending on how you call it.

The EPA tests cars in a controlled environment, and that means running them on a stationary dyno. This means that your Golf SportWagen’s front wheels would be rolling while the back wheels stood still. The EPA even posted a picture of what this dyno testing looks like, which you can see below with the Dodge Ram.

And a car’s stability and traction control systems don’t like the idea of your front wheels speeding along with your back wheels going zero miles per hour, as CR points out.

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The vehicle could otherwise interpret the test procedure as a dangerous situation or malfunction, activating traction control or stability control. By enabling a test mode, the vehicle will be able to operate during the test process. Once the test is complete and the car is restarted, the car reverts to its normal function.

So the VW has a testing mode in its electronic brain that says everything is okay. But don’t get riled up about the test mode, though, CR explains.

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The existence of a testing mode isn’t a red flag. Most new vehicles have a similar setting. The concern here is that the VW models in question performed differently during the EPA tests to meet emissions targets that differ from their performance in the real world. Questions remain about the strategy behind this dishonest approach, and where in the engineering food chain this additional coding took place.

Emphasis ours. Indeed, the EPA said just as much in their initial report on VW’s trickery, all posted right here.

As the EPA states, these diesel VWs really were running cleaner in this dyno mode.

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And it is this dyno vs road mode distinction that lets VW operate cleanly while the EPA is watching, then burn dirty when the regulators turn their backs.

We don’t know the details of Volkswagen’s diesel cheat, but we do know how their cars would recognize to engage it.

Photo Credits: VW (TDI Beetle shot), EPA (Ram on the dyno shot)


Contact the author at raphael@jalopnik.com.