i don't have the manual for my X3 on hand, but this is covered there. the X3 (at least until '06, not sure about '07 and beyond) uses the ABS system (relative tire rotation speed) to drive the TPMS, so there are no separate sensors on each wheel...

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2006 X3 3.0i
Black sapphire with terracotta Montana leather
Premium & sports packages, heated seats, privacy glass, and BMW iPod integration kit
My first BMW - I love it!

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