The Notice of Violation (NOV) [0] is worth checking out if you're interested in the nitty gritty details of regulatory enforcement. It's quite readable and paints a clear picture that EPA believes FCA violated the Clean Air Act. Specifically that they have software/code (AECDs) which were "neither described nor justified". Also interesting that they already set up a short URL for this particular issue. [1]
[0] https://www.epa.gov/sites/production/files/2017-01/documents...