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This warning message is not specific to any one thing, it simply indicates a failure somewhere within the system. It is only intended to inform the driver there is a problem, not to facilitate trouble shooting. This is a very complex system with a number of components that are prone to failure.
There are many things that can cause this error. A failing alternator, a failure of the control module, a bad steering angle sensor, a low fluid level in the system or some other sway bar or sensor issue.
When this warning comes on, the car’s computer logs a record of the fault which has triggered it. So, the only way to determine what has failed is to have the fault memory read from the vehicle using a proper BMW scan tool. Once the faults are read, they will point you to the failure which can be rectified, and proper functionality restored (and the warning turned off).
Given the range of possible causes, there is a range of possible costs. It could run anywhere form £300 to £3000 or more depending on what exactly has failed.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.