I keep asking for technical sources, and you keep making these blanket statements that are obviously untrue, so I don't really know how to continue the conversation.

For anybody else reading this who wants some actual details, here, is a very simplified list of differences. As usual, it's much more complicated than can be conveyed easily in a reddit comment, since the US and EU have different classifications of vehicles (light-duty, heavy-duty, etc.) and also conduct their emissions testing differently. And then there's California.

If you want a quick illustration of how different the standards are, you can look at particulate matter emissions. Euro6 limits PM to 0.008 g/mile, while California is usually 0.08, or 10x the EU standard. I say usually because, if you look at the California link, it varies quite a bit.

tldr, when somebody like /u/Redgrain4 starts yelling something like "You don't have an engineering degree!" it's a good sign they're wrong and trying to provide a distraction.