As Rick says they are an infra-red device and can fail.
However, a common fault is spiders walking across the face of the sensor.
It is often worth taking the cover off and giving them a quick clean or
blast of air (from the compressed air cans used to clean out electric gear)
and re-checking. You may find the one outside that works has a weatherproof
case and small crawly beasties don't get inside.
There should be a small switch inside them that will turn the visible light
off if you don't want them to be seen. There are normally only two wires
feeding them so replacement isn't too difficult - mounting them is often a
greater problem. If it is replaced make sure you get the correct patterned
lens for the area it has to cover.
Guy R Day
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