DEF is essential for the new diesel vehicles. The vehicle’s capacity to travel at a speed of five miles an hour will stop at the point you’ve run out of DEF. The tank should be refilled at least every 5,000 to 6,000 miles. You will learn how this can be done in this video. Take a look at this video right now to learn more about DEF and the consequences if you are out. 88mhnukb7v.