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Did you change the batteries in the base system? if not, don't worry about forgetting programming. Only if the batteries are very old, do they begin to lose capability to hold charge, or maintain a sufficient voltage level when powering the base system box on their own without power (e.g. during a power failure or disconnection.) I think that it's safe to say that, if you have had no power failures or power shutdowns of any duration for some time before the fault with the motion detector started, then the batteries in the base system box are not at fault.
As for the programming of the base system... if its batteries were replaced, that would activate a tamper condition, yes. And you would need to clear that tamper condition, even if the batteries put in were perfectly good and the condition was remedied by that means.
Reset the tamper condition on the main system box, and I would hope that the problem would be solved. (Unless, perhaps, you replaced batteries in the base system box... in which case, also, the tamper condition would need a reset.)
As for whether the motion detector has a fault... check its batteries (if you haven't already done so.) Also make sure a waving bush or a waving wire or cable, is not setting it off ?
Let me know a little more about the problem, and I will advise you further. Thanks.