Temperature cotrol has been working on the highet setting for heat and lowest setting for cold for a year but now it only blows cold air on both settings.

Try replacing the thermostat. As it is probaly sticking

if the temperature of the engine is normal, as in not over hot, not too cool, leave the thermostat alone. there are too many other possibilities, personally look into the blower motor resistor, and pig-tail. hearing the "gates", is the actuator door, which is a good thing, are you capable of flushing the heater core yourself??? that never hurts. let me know if i can help further

Could be a vaccum leak or the air box temp sensor or the control head it self The symptoms you mention either full hot or full cold means the blend door isn't working