Disc will not start, turns over but not start and battery is good.

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Discovery will not start. I tried to jump it hoping it was a door ajar overnight, but not the case. It has 90000 miles on it and has been running well. The transfer box light/sound comes on after driving for long periods of time (over 2 hours). My mechanic (non dealer) tried to figure out where the issue was, but could not get complete diagnostic from it. So it had to go to dealer. Def worried about cost