The oxygen sensor monitors exhaust gas oxygen levels, aiding the PCM in maintaining an optimal air/fuel ratio of 14.7:1. It sends voltage signals fluctuating between 1.0-volt and 0.1 volt when warmed up. OBD-II vehicles have two heated sensors: the upstream sensor in the exhaust manifold and the downstream sensor post-catalytic converter. The latter gauges the converter's efficiency. Issues in the oxygen sensor circuit can trigger diagnostic trouble codes. Before deeming the sensor faulty, other impacting components should be checked. To test, measure its voltage under various engine conditions. If malfunctioning, replace using an oxygen sensor wrench, applying anti-seize to the new sensor's threads and tightening to the specified torque.