I'd buy the Genuine BMW one... it's not terribly expensive for the X5 and it's a part that goes bad often so I'd stick to a new BMW part. I've never had any luck with used heater control valves since they all fail in the same manner.

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2003 X5 4.6 | 2000 540i Touring

Formerly owned:
2004 Range Rover | 2000 M5 | 2000 540i/6
1995 750il | 2003 Mercedes S500 | 2003 540i/6