BMW X3 E83 Key Battery replacement

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Try and see first if the key was simply "forgotten" by the car by reprogramming it:

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The two halves of the casing are glued together. Separate the seam between the upper and lower casing with a sharp utility knife.

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When the seam is open wide enough, slide a pick inside.

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Here is what the inside of the casing looks like. The circuit board has already been removed.

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The circuit board.

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Desolder the battery from the solder pads marked in red.

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If there isn't an insulation layer on the back, then place a piece of insulation tape there.

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Put everything back together. Do not glue the casings yet. Instead, hold them together with a rubber band.