The Oxygen Sensor (O2) measures how rich or how lean the exhaust gas is. This sensor then sends the information to the microprocessor so the processor can make the necessary changes to correct the air/fuel ratio. The process is referred to as a closed loop operation. This mode of operation is important to achieve maximum fuel economy. These exhaust manifolds already had provisions for O2 because of the Lean Burn ignition system. The kit comes with a weld in bung if you need it.