Heat/cold knob not working

As this is a mode issue you would want to look at the number three actuator in this image. It is just above and to the right of the drivers side foot well. Remove the panel under the dash and you should see the actuators above you. Cycle the mode control and see if the actuator moves. If not you could unhook the hose to it and apply vacuum from a hand pump and see if it moves then. If it works with an exterior vacuum supply you will need to check the line for damage or blockage back to the control head. There are also seals in the control head that could be leaking and preventing the vacuum from reaching the mode actuator. Or the programmer could have an issue.