Necessary for use on all 2004 Mustangs looking to change out their midpipe and full length header applications, these 02 Extension harnesses extend the length of your 02 sensors, enabling them to actually be connected to the harnesses that report to the cars ECU whats going on with your car. No check engine lights here. These will work for all Mustangs from 1986-2004, just plug and drive! Sold as a set of two.