The oxygen in the exhaust mix with such elements in the oxygen sensor to produce a voltage output of between 0.1v high oxygen, or lean mixture, and 0.9v low oxygen or rich mixture. The preconverter oxygen sensor, located upstream of the
Catalytic Converter, provides a feed back signal to the PCM as to the amount of oxygen remaining in the exhaust, which the PCM constantly samples in order to adjust the fuel injector pulse width to maintain a stoichiometric air/fuel ratio of 14.7:1 for emissions, fuel economy and power. Unlike the upstream oxygen sensor after the catalytic converter, the so-called second oxygen sensor does not participate in the regulation of air/fuel ratio by the PCM but is used for evaluating the efficiency of the catalytic converter; it has a slower and oscillating voltage signal due to a lack of oxygen. Car manufacturers usually install four oxygen sensors; two of them are located upstream while the other two are downstream; upstream sensors are installed within the exhaust manifolds while downstream sensors are installed behind the catalytic converters. An oxygen sensor does not produce voltage below what seems to be its working temperature of around 600 degrees F; the PCM works in an open-loop fuel control mode relying on data received from other sensors during this time. Electric connections, supply of air, working temperature and unleaded fuel are vital in the good working of this oxygen sensor. The PCM can detect different types of oxygen sensor problems and set DTCs to signal specific problems, and go to open-loop fuel control when a DTC is present. For replacement, it is recommended to make the engine start for several turns in order to ease the process of the oxygen sensor's removal because of the constant contraction of the exhaust pipe. Upstream sensors are reached from the underside of the vehicle and downstream sensors are situated after the catalytic converters. When fitting a new sensor use a clean rag to clean the threads, if reusing the old sensor apply anti-seize compound, however new anti-seize compound pre-coated on new sensors. Turning off the negative terminal of battery for few minutes readjusts the PCM to the new sensor, but it will in the mean time have an impact on the engine performance since the PCM is now in the learning mode of different MPa.