HI I HAVE A 2012 FG XR6, 20000KS AGO FITTED PACEMAKER EXTRACTORS AND HIGH FLOW CAT AND DONE THE REST OF THE PIPE WORK MYSELF.
HAS TWO SENSORS , ONE AT THE BOTTOM OF THE EXTRACTORS ONE AT REAR OF CAT CONVERTER.
LIFE HAS BEEN GOOD EXCEPT 2000KS AGO THE ENGINE WARNING LIGHT CAME ON THE REAR SENSOR HAD RUBBED ON THE GEAR BOX REPLACED IT (SENSOR) SHIFTED EXHAUST AS TO GIVE MORE ROOM , ENGINE LIGHT STILL ON TOOK TO FORD REPLACED FRONT SENSOR, LIGHT STILL ON, THEY SAY REPLACE EXHAUST TO STANDARD AND LIGHT WILL GO OUT.
DOSE ANY HAVE ANY INPUT ON THIS PLEASE.
BRUCE

The exhaust has stuffed up your O2 monitoring due to changing the lag time of the O2 correction to learn beyond the limit. The PCM detects this as a fault and raises the lamp.

To get rid of the lamp you need to flash the ECU to disable the rear sensor or ignore the DTC that is raised when the O2 system learns a long transit time. Remember that your car will never pass emissions with the new exhaust, that is the main reason the lamp has come on and ford are correct in saying you must put the stock exhaust back if you want it to go away.

Most tuning workshops should be able to do a flash to delete the DTC warning for you cheaply as it only takes 20 minutes.