Thanks to all of you for the great information and feedback, I appreciate it. As mentioned, expansion tank and sensor have been replaced, system full and bled properly. Message continues. I will check voltage at sensor connection and report back with updates. It sure if baffling/annoying, but I'll figure it out one way or another! Thanks again.

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2006 X5 4.8is
LeMans Blue over black
Style 168 rims
Blackout grills and hood vents
Aux jack