Waiting for a problem with the battery is how you are much more likely to lose the EWS synch. If you don't want to spend any more money on the key fobs then at least make sure that the manual lock is working and dependable.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)