Get thee to a dealer. I had one go, they just replace it. It is an expensive part, but I assume you are still under warranty.
'05 E90 325i 6sp Man SG 118,032km on the odo (retired) | '06 E90 325i Step SG 121,021km on the odo (retired)
'08 E90 325i LCI Step SG 130,024km on the odo (retired)
'13 D30 328i M-Sport Step | Estoril Blue | Black/Aluminimum | Pro Nav | Xenon HBA | Lane Change Assist 1,760km on the odo