These plugs, supplied in pairs, are essential when installing an aftermarket exhaust that does not allow for the factory O2 sensors to be retained, if the exhaust flow is substantianally altered or even if the airbox or air cleaner is modified enough to alter the bikes readings. Modern Harleys do not allow air/fuel mixture adjustments without upsetting the bike's internal diagnostics, but these units provide a contant 'happy' signal to the bikes electronics. Price is for a pair and are available for virtually any 2007-2011, and 2012 and later models. Image shows the 2012+ units. I can also supply single units for just air cleaner sensor bypass and a unit that allows the exhaust butterfly valve to be removed.