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This isn't apparent unless you read the article I linked, which is long. I don't expect you to.

The main violation appears to be emitting excess amounts of NO2, a pollutant, and some particulates.

My understanding (again, someone please correct me if I'm wrong or missing something) is that NOx emissions are mostly eliminated by injecting DEF (urea) into the exhaust stream, and also that NOx emissions are the big issue with dieselgate. And again, from what I read, the whole reason they didn't want to do it right was that consumers would have to refill the DEF tank too frequently, and that automakers thought people are too stupid for that and would run it dry, so they wanted it so that the dealership would refill it for them at their regular service interval, which is 10-20k miles. So as I said before, the fix should be easy, inject a lot more DEF, enough to actually eliminate the NOx per the standards, and just force consumers to refill the DEF far more frequently.

Looks like I was way less informed, thanks for the context!

It appears your analysis is right.

What would happen if the DEF was allowed to run empty?

The way they are now, I'm not sure. High NOx emissions I'm guessing, plus a warning light on the dashboard. But my proposal is to make the car stop operating if the tank is empty, just like it stops operating if your fuel tank is empty, and just make sure the dashboard light (or better yet some alphanumeric display or infotainment system display) gives ample warning to the driver that they need to refill it. I see Blue DEF sold in gas stations all the time, so it's not like it's hard to acquire.

And again, I'm not even sure I'm that well informed, I could very well be mistaken and over-simplifying things. If that's the case, I hope someone will correct me, but so far no one's said anything, but that's why I put that disclaimer in there several times.

This is how heavy truck diesels with DEF work, at least. Warning light at 10% full, flashing warning light at nearly empty, and if the tank runs completely empty, the vehicle is limited to 5mph until it's refilled.

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