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.