An oxygen sensor is a galvanic battery that produces a low voltage as a response to the amount of oxygen in exhaust gases and supplies necessary input to the feedback loop with the Powertrain Control Module (PCM) to keep the optimal air/ fuel ratio of 14.7: 1 to achieve optimum
Catalytic Converter operation. Car models with On-Board Diagnostics II (OBD-II) have two oxygen sensors (pre- and post-catalytic) that measure the efficiency of the converter by measuring the concentration of oxygen in the exhaust gases. In four-cylinder models, there is normally one upstream and one downstream, Catalyst Monitor Sensor, whilst in V6 there are more than one upstream sensors and one downstream catalysts. All oxygen sensors are also heated in order to increase the time taken to warm them and any mishandling during service should be avoided when attaching the pigtail permanently and the electrical connector permanently and also to prevent any contaminant coming in contact with the sensor. When changing an oxygen sensor, one may recommend warming the engine to facilitate the removal, use of anti-seize compound on the thread, in case of reusing an old one, and proper installation and torque. Once the battery has been reconnected the PCM will need to relearn its idle and fuel trim strategy to achieve maximum performance.