chevy1500z71

so im getting code 28- Gear Switch Diagnosis (3rd Gear ) and i really dont know what its getting the code from or how to fix it? what eactly does the code mean, i mean what sensor is it reading from, and how would i go about fixing this problem whatever it may be. i have noticed that it didnt want to go into overdrive once, but for the most part its been acting normal, good firm shifts, all through the gears. it only has about 25k miles on it. it was built for 700hp and is supposed to be a really nice transmission so whaty could be wrong? it is 4l60e by the way.

i was about to start tuning my truck tomorrow but now i need to worry about this! always something lol

any help would be appreciated. thanks.