On ODB-II vehicles (96+), the ECU has two O2 sensors for diagnostic purposes. The first O2 sensor is located before the Catalytic Converter and is responsible for all the Air/Fuel tuning and is imperative to the operation of the vehicle. The second O2 sensor is there to make sure the catalytic converter is doing it's job properly. By Utilizing the Torque Solution 45 Degree O2 Sensor Extender to move the second O2 out of the exhaust stream on modified vehicles it will reduce / prevent the risk of throwing a CEL.