Ford recently announced a delivery hold on certain (not all) 2015 Mustang GT models for an inspection and replacement of catalytic converters.
According to the announcement (read the full document), this affects certain 2015 model year Mustang vehicles equipped with 5.0L engines built from February 11, 2014 through October 3, 2014. In some of the affected vehicles, one or both catalytic converters may have been built incorrectly and may affect the performance of the emission control system.
If your vehicle is affected by this issue, it will appear HERE.