2005 gmc envoy check engine light

We recently bought a used envoy and have had problems since day one. After driving it off lot gas light was on, filled tank and light continued to stay on. We found out it was a problem with the fuel gage which i guess is a common problem. Now engine light comes on for 2-3 days at a time and then goes off for a day or so. When gage problem was fixed it was supposibly hooked up to a diagnostic machine which showed no other problems. Obviously not true. Truck seems to drag and not change gears like it should, also interior roof lamps flicker off and on sometimes. Anybody have any advice as to what may be the problem. Sadly no longer under warrenty, but did buy from family member which will fix it.

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