An Aftermarket O2 sensor to save you money that already has the correct harness installed! Previous Aftermarket sensors left it up to you to cut, splice and solder the harness to the wires, which is troublesome. This is completely plug-n-play, uses O2 sensors that are the same spec as Mazda OEM, and have been completely tested. A malfunctioning O2 sensor doesn't send correct data to your Mx5's ECU and causes it to run poorly - lean, which leads to detonation; or rich, which wastes fuel and causes excess emission. In some states you could fail the smog test needed to update your registration.