You've probably used an automatic paper towel dispenser before, or walked into a room and the lights turned on automatically. That's because of Passive Infra-Red (PIR) sensors. You also more than likely know the frustration of waving your hands inside of a room that suddenly goes dark or standing in front of an automatic door that refuses to open. The sensor failed. But the question is why, and can it be fixed, or does it need to be replaced?