The sensor has two wires in, but the light comes on when the sensor touches the disk and earths through it.
So really you only need one wire to detect the pad wear.
What I think is that the two wires are a loop so that if it earths on the disk it puts the light on but if it detects a break in the loop it knows the sensor is broken or disconnected and puts the light on.
Now I guess that when it wears down a bit it separates the wires by wearing away the contact that joins them. Hence putting new pads on when a sensor is too worn leaves the light on.
I guess........ seems a logical way to do it.
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