automatic A/C -heat control

the climate control in my 2004 tahoe does not work in automatic - whenever you select automatic it will only blow cold air - the A/c compressor runs and cold air comes out- you have to switch it to manual and select the temp to 90 degrees to make the heat work. I have replaced the climate control - control board ( the one in the dash with the knobs and selector switches and screen ) this did not fix my problem - I have been told it is an actuator or a blend door control or a climate control resistor, I called the chevy place and they said there is no listing for a climate control resistor. but how can it be an actuator if it will work when you turn it to 90 degrees. I really dont understand how to fix or troubleshoot this problem thx for any help you can give