Full heat with temperature set at 61 degrees

My truck listed above is a 2500 HD, Duramax, crew cab, LT trim.
Cooling works, heating works. Problem is with a set point of 61 degrees or higher I get full heat with the air being sent to floor vents, high speed fan and discharge air temperature as hot as can be! Once set point is moved to 60 degrees air discharge moves to dash vents and goes full cold. I did remove cabin temperature sensor from headliner, cleaned and verified fan operation. Unfortunately, I did not check resistance value of sensor when I had it out.
Any advise would be greatly appreciated!