The other issue the OP has in this scenario is...he's not considering that his two X5s have two different engines (DME/ECU). The 05 has the N62 double VANOS engine...whereas the 01 has the M62TU single VANOS engine. I doubt that the DME/ECUs will just swap over without some major issue.

It would have been simpler (and cheaper) just to buy a new key from BMW. If money is tight and the $100 + remote key is too much...there's always the other non remote key(s) that cost much less. The earlier models have the choice of an illuminated key, spare key with metal blade, or plastic spare/wallet key, or the limited use VALET key (only works the door & ignition). The later model years (10/2003-up) only have the remote keys and the plastic VALET key.

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• 2015 f06 650ix GC (Moonstone exterior/Cohiba brown Merino Leather interior)
• 2014 f15 X5 50i (Space Gray exterior/Mocha Nappa Leather interior)