Try reprogramming the key, I done this a few days ago for a friend and it worked perfect!

Get in to the X5
Close all the doors and hatch
Put key in ignition turn to position 1 then turn back and take key out
Immediately after taking the key out press the hatch button once,
then hold down the "door open" button on the remote and at the same time press the "door lock" button on the remote 3 times and release all buttons,
The doors should lock and unlock which is the sign your key has reprogrammed correctly. If the doors do not lock unlock you may have not done the steps quick enough or correctly, failing that, the key may need replaced.

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