I know on a lot of these trucks there is an orange wire that runs down from the gear selector down through the column that controls tow/haul. This wire has a tendency to become broken. Not sure if this is your problem but somewhere to start looking.

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thank you i will look. the truck has no codes. no engine light on either.

 

When mine broke it did not drip the CEL either.

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when i select manual mode stays in 6th gear on dash but i fell it shifting.

My 2013 does this too, it's normal I pretty sure. Try putting it in M2 and see if it only shifts once to second. It should hold second, but still take off in 1st. In M6 it should still shift through the range of gears. Think about it, if it tried to start off in 6th gear while in M6 you'd be burning that trans up in short order.

 

when i put it in M it stays in 6 on dash display, the transmission shifts throught the gears i just cant select which gear i want. I was told by Chev dealer that if i put it in 4th it will up shift and down shift up and down by it self.

 

Now I gotcha... Yeah if the +/- and trailer button are not working, then I'd be looking under dash for loose or broken wires as Travis suggested.

Good Luck.