A method of controlling a vehicle heating, ventilation, and air conditioning (HVAC) system. Outside or ambient temperature, evaporator temperature, and coolant temperature are measured with sensors. These values, along with a TAO, which is a calculated value of outlet temperature in the vehicle, are sent to a controller. The controller determines a predicted heater core outlet temperature based upon the coolant temperature. Based upon the outside air temperature and coolant temperature, the controller will decide whether to use the previously determined estimated heater core outlet temperature or to substitute an alternate value for the heater core outlet temperature to determine a position for the air mix door. After calculating the air mix door position, the controller signals an air mix door movement device to adjust the air mix door to the required position. Position of the air mix door affects the air temperature inside of the vehicle.