Phil,
Your fault code P1647 is defined as 'HO2S bank 2 control module open or shorted'.This is a confusing definition as the 'control module' means the sensor itself and I think it is a titania type toggling between 5v and 0v.On the X type the front bank is bank 2.I'd say you have a good case for renewing the front precat sensor,and genuine is not too expensive.