When you install a set of BBK Performance long tube exhaust headers 1633 or 16330 on your 2015 -17 Mustang GT 5.0 the O2 sensors are relocated on the header that is no longer in the factory location and the factory O2 sensor connectors will not reach to connect any longer.. These extension harnesses and bolt kit is required when Installing BBK 1633/16330 or 1856/18560 Headers on a 2015-2017 Mustang 5.0 - All harnesses feature OEM Factory connectors to allow simple plug in installation to the factory wire harness so that the relocation is super simple and does not require any splicing of the factory harness at all. (Kit Includes 2 Harnesses and 5 Bolts for installation)