In order to maintain optimum efficiency of the catalytic converters, the system endeavors to provide the ideal air-fuel mixture ratio (Lambda = 1) for combustion.
To analyse the composition of the exhaust gas, oxygen sensors (control sensors) upstream of the catalytic converter are used. They are mounted on the exhaust manifold immediately before the catalytic converters.
The oxygen sensors measure the residual oxygen in the exhaust and send corresponding voltage signals to the digintal engine module (DME). Here, if necessary, the mixture composition is corrected accordingly in that the injection timing is varied. Depending on the operating status, oxygen sensor values greater or lesser than Lambda=1 are the aim. If the oxygen sensor fails, the DME controls injection on the basis of permanently stored substitute values dependent on engine load and speed.
The oxygen sensors located after the catalytic converter serve the purpose of monitoring the control sensors and efficiency of the catalytic converter.
For each cylinder bank, there is a sensor before and a sensor after the catalytic converter.
The Lambda sensors after the catalytic converter are the familiar jump sensors (jumps in voltage at Lambda = 1).
The oxygen sensors upstream of the catalytic converter are continuous-detection sensors. They measure the oxygen content both when the engine is running with a rich mixture and a lean mixture and return an appropriate signal. Because of that method of operation, these Lambda sensors have 6 instead of 4 pins in the connector.
The oxygen sensors upstream of the catalytic converter need to be at a temperature of approx. 750 degrees C in order to operate (350 degrees C in the case of the sensors downstream of the catalytic converter). For that reason, all oxygen sensors are heated. Sensor heating is controlled by the DME. With a cold engine, the heating system is inactive, as the condensation water present would destroy a hot sensor due to thermal stresses. This is why the oxygen sensors only become active shortly after engine start-up. Initially, the sensor is heated with reduced heat output so as not to subject it to thermal stress.
The operation of the Lambda oxygen sensors is monitored. Oxygen sensor malfunctions, e.g. caused by using leaded fuel, are detected by the DME. The sensor signals before the catalytic converter are checked against the signal after the catalytic converter. The condition of the sensors is checked by means of special temperature models.
Any faults that occur are stored in the DME fault memory. The sensor concerned can be identified from the stored fault code. The fault codes also distinguish between sensor signal and sensor heater.