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 assess the exhaust-gas composition, Lambda oxygen sensors (control sensors) in front of the catalytic converter are used; these are fitted on the exhaust manifold.
The Lambda oxygen sensors measure the residual oxygen in the exhaust gas and send corresponding voltage values to the control unit. 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. In the event of a Lambda oxygen sensor failing, the engine control unit assumes emission control with a programmed substitute value.
The oxygen sensors located after the catalytic converter serve the purpose of monitoring the control sensors and efficiency of the catalytic converter.
On a 4-cylinder engine, a special arrangement is used: The post cat. sensor of bank 1 is arranged between the pre- and underfloor catalytic converter; the post cat. sensor of bank 2 lies behind the underfloor catalytic converter.
For each cylinder bank, there is a sensor before and a sensor after the catalytic converter.
The operation of the Lambda oxygen sensors is monitored. Malfunctions of the oxygen sensor, e.g. caused by using leaded fuel, are detected in the engine control unit. The sensor signals before the catalytic converter are checked against the signal after the catalytic converter. The state of the sensors is checked using temperature models.
The Lambda sensors after the catalytic converter are the familiar jump sensors (jumps in voltage at Lambda = 1). The Lambda oxygen sensors before the catalytic converter are permanent sensors, i.e. they measure the oxygen content both in the rich and lean range and issue a corresponding signal. Due to a different measuring principle, these Lambda sensors have 6 instead of 4 pins in the connector.
As a temperature of approx. 750 degrees C is required for operating readiness of the Lambda sensors before the catalytic converter (350 degrees C in the case of sensors after the catalytic converter), all Lambda sensors are heated. The heater is actuated by the engine control unit. 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.