The O2 sensor is installed in the exhaust pipe just after the collector. This component tells the computer how much unburnt fuel is in the exhaust so that it can adjust the air/fuel ratio to the ideal 14.7:1. This ratio provides the best power and economy in most situations. You can have the O2 sensor bung welded in by a muffler shop before you begin the conversion if you don't have welding equipment. This is the only welding required.