I had the same thing and it was my sensor pad on my passenger seat. Bought a bypass sensor and seatbelt emulator (bought as a kit on Amazon or ebay and was sent from Ireland). Easy install and works flawlessly. Airbag will still deploy but will regardess if someone is sitting there or not.

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2006 BMW x5 e53, M54, steel grey, black leather, pano roof, cold weather package, avic-z150bh head unit with JL Audio amp and speakers.