Mine threw out codes for misfires on all cylinders, O2 sensor limits, O2 adaptation limits. I got a small oil leak to the valve cover gasket too and another (bigger) to one of the hoses coming to the OS, however once again I'm driving in hot climate so I didn't experienced some major issues described by some owners living in very cold areas.
Depending of your mileage and how old your X is, replacing the OS is one of the most useful preventive maintenance...