In order to maintain optimum efficiency of the catalytic converters, the system tries to provide the ideal air-fuel mixture ratio (Lambda = 1) for combustion.
To analyse the composition of the exhaust gas, Lambda oxygen sensors (control sensors) upstream of the catalytic converter are used.
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 Lambda oxygen sensors located after the catalytic converter serve the purpose of monitoring the control sensors and efficiency of the catalytic converter.
The operation of the Lambda oxygen sensors is monitored. Malfunctions of the Lambda sensors, 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 before and after the catalytic converter are the familiar jump sensors (jumps in voltage at Lambda = 1).
As the oxygen sensors upstream of the catalytic converter need to be at a temperature of approx. 350 degrees C in order to operate, all Lambda sensors are heated. This heating is activated 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.