They can hook it up to INPA and diagnose the fault code, then go from there.
I had it happeedn one time with the 4.8iS. Turn out it's the battery (we know it needs to be replaced base on the PPI) I clear the fault code after replacing the battery and all is good.

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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles) http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
01 360 Novitec Spider F1 (26k miles)