I drove back from the Rally in one shot to Dallas. Several hundred miles from home, I got the warning of Low def. Since I did have a box of blue Def in the back and it was late at night I wanted to see what would happen if I ignore the warnings.
1. When it was really low, it told me that my speed was now limited to 69 then 65 MPH. So I kept going.
2. Then when it was out, the display said I was out but then also said I had 400+ ml left. So I kept going
3. The 400ml kept counting down, about 1ml per mile.
4. Looks like I could have gone another 400 miles after I was warned I was out. But I arrived home before that happened.
Moral of the story, I no longer will be stressed out when the display says I have Low Def. This is for GMC.