The O2 sensor on the Catalytic converter is the secondary O2 sensor. The one on the header (nearer the engine) is the primary. The secondary O2 sensor is used to check that the catalyst is working properly. The secondary sensors reading is compared to the primary sensors reading by the computer to determine if the catalyst is converting the residual oxides in the exhaust. So a code indicating catalyst problems can be caused by the catalyst, or a faulty secondary O2 sensor.