Not Recommending This, But Curious
Funf, you've helped me before and you're active on the DEF related forums so maybe you have an idea regarding this speculative question.
Imagine that for some reason(s) a diesel owner has ignored the warnings and the mileage count down, finds himself in the middle of nowhere with the urea tank empty and the warning stating that his X5 will not start once he shuts it down. Could you fill the tank with water and would the vehicle start, or is there some advanced sensor that can determine that it is not merely the level of fluid but tests for ammonia content? In an emergency could this procedure work and would there be any damage to the system?
Might not even be an emergency, maybe the local dealer says they won't top off the tank to get to the next oil fill and the owner just doesn't want to pay for the solution and so he wants to get by for the next 1K miles or so.