To default the slimline door/window sensor:
1) Remove the cover using a small flathead screwdriver.
2) Remove the battery; press and hold down the tamper switch(small black button near battery).
3) With the tamper switch still held down, insert the battery and release the tamper switch.
4) The LED will begin to flash once the sensor is defaulted to signal that is ready to be added to your system. Click Here for a guide to adding sensors.