Might be: Poor adjustment of the sensor, battery low on wireless sensor, broken/damaged wire on wired sensor, input of alarm panel defective, for starters.
If it says "back door open", it means the sensor for that door is not making proper contact and it thinks the door is not closed, even if it actually is. If you do not know how to fix that, then you arrange to get a security expert to come out and service the system.
Or you disable the system and get a really yappy dog that alerts you when someone tries to open a door or window.
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Sensor on the back door is defective or not making good contact with the magnet.
someone has messed with the sensor on that back door becouse they are planning on coming in through it, at a later date.
Might be: Poor adjustment of the sensor, battery low on wireless sensor, broken/damaged wire on wired sensor, input of alarm panel defective, for starters.
Perhaps the battery on the back door sensor is in need of replacing.
If it says "back door open", it means the sensor for that door is not making proper contact and it thinks the door is not closed, even if it actually is. If you do not know how to fix that, then you arrange to get a security expert to come out and service the system.
Or you disable the system and get a really yappy dog that alerts you when someone tries to open a door or window.