Yes, the heater control valve (not duo-valve - this is in some other
models) is located under air intake grille adjacent to blower.
It is vacuum controlled through the two temperature knobs, so it opens
whenever one of the temperature control knobs are not in fully ccw
position, or if there is no vacuum present due to a defect in the
The temperature is then adjusted by mixing hot and cold air through
flaps mechanically linked from the temperature control knobs.
Your symptom indicates the heater control valve is working (heating is
off when both controls are fully ccw and on when not), but probably air
flaps are stuck in the hot position.
Check the linkage from the controls.