Am working on my neighbors car, it is a 2004 S60 with ECPS, have replaced steering rack with used one and the power assist worked great for a while now the steering is back to notchy and hard turning. Have done a hard reboot to the module and it works great for awhile now it is back doing the same thing. Can anyone tell me where the control unit is for the steering so I can Ohm the wires to see if there is a sporadic short or if the unit needs replaceing.

 

Might be corrosion at the upper and / or lower inside knuckles inside the of the column.