Heat need heat!!
 

My truck is having heating issues! I read that if move it to cold from hot you should hear a fliping noise down in the center above the tanny. But i hear something when i move the switch it from vent, feet, face ect.... But it dose have warm air when its on 1 or 2 but when i crank it to high nothing but cold air. the coolant is full and i dont think it needs to be flushed it looks clean and i flushed it at 130,xxxx now im at 180,xxx. what could be my problem. 1999 z71 ext 1500

Sounds to me like heater core is plugged. Could also be a thermostat

wouldnt the truck over heat then?

no the truck would not heat could have bad thermostat or block heater core

feel the hoses for the heater core at the firewall (engine side ) to see if incoming and outgoing are hot. if yes you have a thermostat issue likely. (stuck open)

I am have the same problem, on my k1500 5.7 v8 it will not blow warm air! I changed the radiator (leaked) the thermostat, the heater core, the blend door moves and the water pump (flow was not constant). Still blowing cold air! Can't figure it out

looks like your last bet might be the controls....

The controls? Are you talking about the board with fan speed, temp, and air direction on the dash? I forget what it's called.

I had the same issue with mine turned out to be the heater core...Couldn't change it myself, but a good friend of mine did!

Friend of mine had a problem with heat. Turned out to be air locked. Took heater hose off and pushed water through the system with a garden hose. Had to remove the thermostat to do so. But u will lose your antifreeze. Night drain it first if it clean. Just a thought!