* Faulty blend door actuator: This is a common issue with Mustangs of this era. The blend door actuator is responsible for controlling the flow of air from the heater core to the vents, and if it fails, you will not be able to adjust the temperature of the air coming out of the vents.
* Faulty temperature control switch: The temperature control switch is responsible for sending a signal to the blend door actuator to adjust the temperature of the air. If the switch fails, the actuator will not receive the correct signal and will not be able to adjust the temperature.
* Faulty wiring: The heater control assembly is powered by a set of electrical wires. If any of these wires become loose or damaged, it can prevent the assembly from receiving power and functioning properly.
* Faulty control panel: The control panel is where you interact with the heater control assembly to adjust the temperature and fan speed. If the control panel fails, you will not be able to control the heater.
Here are some additional things you can check to troubleshoot the issue:
* Make sure that the heater control assembly is receiving power by checking the fuse for the assembly and testing the voltage with a multimeter.
* Check the wiring for the assembly for any loose connections or damage.
* Try manually moving the blend door to see if it is stuck or obstructed.
* If you are still experiencing issues, you may need to replace the control panel, temperature control switch, or blend door actuator.
