This extension is not mandatory for our 4-2-1 ND header. The o2 bung on that header is about 1/3rd of the way upstream from the lower flange, and we find that we can usually get the o2 sensor to reach there by just cutting back the wiring loom a bit to get some extra length. However, if you want to avoid that work you can use this extension to make it a simple plug and play install.

This extension IS ALREADY INCLUDED with our 4-1 ND header, since the o2 bung on that header is all the way down at the lower flange so an extension is necessary to reach there. If you bought our 4-1 header, you already get this extension included with the header, don't buy it again unless you need a replacement!

Care should be taken when installing and routing the wires to make sure the wiring is kept away from the hot header.