Ok I been reading on here about this code... And they say it can be o2 sensor but I dont think so... I think it is the DPFE sensor... Delta Pressure Flow Emissions or whatever... Suppose to be common on mustangs... I put two used o2 sensors on the rear with mil eliminators so I dont think they are the problem... It seems like there is bit of a putter in exhaust maybe... So definatley not rear o2's cause they do not effect the way the car runs... Maybe its bad EGR... I am considering taking it off and cleaning...