what can be the cause of power steering failure?

Hello, This is common on the 03-06 Z4. The steering rack is electronically controlled, so the control module is cancelling power steering. Most people complain about the steering being 'sticky', while others complain about power steering failure at certain ranges. There is a fix for the power steering rack, and the replacement costs a ton. Have it diagnosed with a professional scan tool before deciding to replace anything.