Oxygen Sensor After Catalytic Converter

Oxygen Sensors and Catalytic Converter

Function
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 assess the composition of the exhaust gas, oxygen sensors (control sensors) upstream of the catalytic converter are used. The control sensor are bolted onto the exhaust manifold.

The Lambda oxygen sensors measure the residual oxygen in the exhaust gas and send corresponding voltage values to the engine control module. In the engine control module, 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 module assumes emission control with a programed substitute value.

The oxygen sensors located after the catalytic converter serve the purpose of checking the function of the control sensors and efficiency of the catalytic converter.

Diagnosis
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 module. 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.

Design
The Lambda sensors after the catalytic converter are the familiar jump sensors (jumps in voltage at Lambda = 1). The design of the Lambda sensors in front of the catalytic converter varies depending on the exhaust requirement. They are either the familiar jump sensors or constant sensors. These constant sensors measure the oxygen content both when the engine is running with a rich mixture and a lean mixture and return an appropriate signal. Due to a different measuring principle, the constant Lambda sensors have 6 instead of 4 pins in the connector.

Heating
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 module. 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.