2011 GMC 3500, 15,000 miles. 'three weeks ago DEF message, went away before I could read it, something about exhaust ..... see owner's manual. Message went away but engine light on. I filled with DEF, CEL light remained on for a few days then went out. All good, or so I thought. Next came DEF quality poor. That too went away after a few days. Then just two days ago the CEL came on again, no message, still on. I can no longer trust the truck. From the same GMC dealer I have bought 4 vehicles, a 2004 Yukon, 2004 Sierra 2500 (gas), 200t Sierra (gas), and this 2011 Sierra 3500 D-Max. I can't risk DEF problems causing me to limp when I'm 1,000 miles from home. I loved the truck until now, but it will soon be replaced with a RAM 3500 and, yes, a manual tranny.