Problem on Visonic Powermax plus intruder alarm ?

My 5 year old Visonic Powermax plus home alarm has been triggered on 3 times in the past 3 months for no apparent reason. After disarming the alarm the control panel displays Ready and Memory alternatively.Memory interrogation displays back door (which has a door contact). Upon arming and disarming alarm control panel displays Ready i. e ready to arm. Any suggestion on possible faults and remedies will be greatly appreciated