Your Oxygen Sensor is located in the exhaust. Vehicle after 1996 will have one oxygen sensor before the catalytic converter and one after. The oxygen sensor before the converter is used by the ecm to decide if the vehicle is running rich or lean and determines how much fuel the engine needs. The oxygen sensor after the converter is used by the ecm to determine if the the converter is doing its job properly. To properly test an oxygen sensor it is you may need to use a scan tool.