Oldkeywest91, the DEF system is not like your fuel tanks, that you can run down to near empty. If you do, you will need to get towed and the red engine symbol will come on. I get about 3000 miles on a tank of DEF. I have indicator lights near my fuel gage. 4 green lights full, 1 green light ok, turns to yellow - should have filled it at 1 green, yellow turns to red, don't stop the engine, will not restart, got about 400 to 500 miles (per Freightliner) till shutdown and having to get towed in.
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Finally completed all State visits in August 2011 with a trip to North Dakota.