by Shruti Vaidyanathan, Senior Transportation Researcher

On September 18th, the Environmental Protection Agency (EPA) slapped the Volkswagen Group with a notice of violation (link is external) of the Clean Air Act for circumventing EPA emission standards for nitrogen oxides (NOx). Volkswagen has now acknowledged using so-called “defeat devices” on its diesel cars that turn on full emissions control technologies while in testing mode but allow the vehicles to emit nitrogen oxide levels up to 40 times the certified level during normal operation. In the United States, Volkswagen will be forced to recall half a million vehicles, including the diesel versions of the popular Jetta, Jetta Sportwagen, Beetle, Beetle Convertible, Golf, Golf Sportwagen, Passat, and Audi A3 models for model years 2009 to 2015.

In response to these events, has suspended Green Scores for the affected vehicles until further notice, since these scores are no longer reasonable estimations of the environmental impact of the Volkswagen diesels. Volkswagen’s diesel cars have performed well on ACEEE’s annual rankings since 2009, hovering just below our list of the top twelve “Greenest” vehicles with Green Scores in the high 40s. However, a 40-fold increase in on-road NOx would mean that these vehicles did not deserve those high Green Scores…

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ACEEE: The American Council for an Energy-Efficient Economy