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Q:

my alarm is malfunctioning, often going off on its own- on 1999 Porsche 911

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
car has 145000 miles. recently alarm malfunctioning and emitting lite on dash not appearing at all.
(1) Answer
It could do this for many reason which requires an OE diagnostic computer, but I'd start by checking the battery with a good battery tester.
This would be the first and cheapest test.

Zee