GMC SIERRA HEATER NOT WORKING

My Heater Will Only Blow Heat If I Put The Temperature To Above 86 Degrees. Anything Under That Is Will Blow Cold Air.

Truck Listed Above V6 3.5, Heater Vent Value Cabin Sticks Sometimes Open Had Heat. Sometimes Close No Heat. Only 196,000 Kms.

The Arrow On The Temp. Control, Flash All The Way Across, Won't Adjust Heat. I Have Replaced The Control Unit And It Does The Same...

Heater Blowing Cold Air All The Time. Replaced Climate Control Board. Blend Door Actuator And Mix Air Actuator. Thermostat. No Bubbling Or...

Heat Set To 72 Degrees; Truck Will Start To Get Warm Then Starts Blowing Cold Air. Turn Heat Up 2 More Degrees And Starts To Get Warm Then...