2006 Chevy Impala COOLING SYSTEM

Just a minor correction, the Hill Descent Control is used in first or reverse, not fourth. That being said, the most likely problem is either a failed tranny or possible trans sensor. Given the fact that you are fed up and possibly looking to trade, I would support you going that route. If you decide to keep it, it requires having the codes read to determine what action should be taken next (replacement of sensor(s) or transmission replacement). Good luck.