I had an issue similar on my spare key. I reinitialized it and it worked well for about another year. Now I'm not able to get it to work any more. There is a thread somewhere here on how to initialized.

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Current
2006 BMW X5 4.8is Dinan S2 Black on Black

Retired
2004 BMW X5 4.8is Black on Cream
2001 Bmw X5 3.0
2006 Bmw 530i
1996 Bmw 328is
1994 Toyota Landcruiser
1989 Bmw 535i