Various fuel trim and oxygen sensor limit fault codes can be traced to an internal vacuum leak at the oil separator.
Car Problem Reports
Porsche Boxster Internal Leak at Oil Seperator Can Cause Fuel Trim and Oxygen Sensor Limit Faults
Porsche Boxster Problem
Visitor, 2003 Porsche Boxster, 43,500 mi
Oil separator failed leading to oil in the 02 sensor caused check engine light to illuminate