START VALVE OPEN light only indicates valve is not in the closed position, sensed by a position sensor.
Interesting information IFixPlanes and this is for the classic and not the NG - I think. Many pilots will back the start valve open light up with a quick glance at the duct pressure. If duct pressure does not return to prestart value then you might have a start valve open problem. Abnormal duct pressure is not as easy to detect when you move quickly from one start to the other but I think it is ok as long as you don’t have a MEL write up of the start valve open light.