Seems like VW may not have been alone: https://www.engadget.com/2017/05/23/us-sues-fiat-chrysler-over-emissions-cheating/
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US sues Fiat Chrysler over diesel emissions cheating
It supposedly used software to mislead regulators.
[...] the US Department of Justice has sued Fiat Chrysler for allegedly using a combination of defeat devices and software to trick regulators into thinking its 3.0-liter EcoDiesel engines are... well, eco-friendly. Reportedly, 2014-2016 model year Ram 1500 and Jeep Grand Cherokee vehicles (104,000 in total) violated the Clean Air Act by meeting emissions rules in EPA testing, but spewing "much higher" nitrogen oxide levels in certain everyday driving situations.
The case would force Fiat Chrysler to fix the affected vehicles and demand other penalties, possibly including a fine. The EPA previously warned that a fine could climb as high as $4.6 billion. That pales in comparison to VW's $14.7 billion settlement, but it's not exactly a trivial amount.
[...]
There could be more trouble coming, if not necessarily in the US. At the same time as the Justice Department was filing its lawsuit, German officials were searching Daimler offices as part of an investigation into diesel fraud. If they find evidence of wrongdoing, that could indicate an industry-wide attempt to play fast and loose with diesel emissions, not just one or two outliers.
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US sues Fiat Chrysler over diesel emissions cheating
It supposedly used software to mislead regulators.
[...] the US Department of Justice has sued Fiat Chrysler for allegedly using a combination of defeat devices and software to trick regulators into thinking its 3.0-liter EcoDiesel engines are... well, eco-friendly. Reportedly, 2014-2016 model year Ram 1500 and Jeep Grand Cherokee vehicles (104,000 in total) violated the Clean Air Act by meeting emissions rules in EPA testing, but spewing "much higher" nitrogen oxide levels in certain everyday driving situations.
The case would force Fiat Chrysler to fix the affected vehicles and demand other penalties, possibly including a fine. The EPA previously warned that a fine could climb as high as $4.6 billion. That pales in comparison to VW's $14.7 billion settlement, but it's not exactly a trivial amount.
[...]
There could be more trouble coming, if not necessarily in the US. At the same time as the Justice Department was filing its lawsuit, German officials were searching Daimler offices as part of an investigation into diesel fraud. If they find evidence of wrongdoing, that could indicate an industry-wide attempt to play fast and loose with diesel emissions, not just one or two outliers.