Just went through kinda the same problem. My alarm would go off only when the temps in summer got hot. Worked fine all winter, just the end of last summer and with in the last month.

I just replaced the hood switch ($17) and it seems fine (3 days). But before the repair my siren stop working. Still trying to figure out if it's a programming issue or do I have to replace it ($149 dealer only).

Good luck

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2001 X5 3.0
1989 325is
2005 Altima V6 (wife)