In order to maintain optimum efficiency of the catalytic converters, the system endeavours to provide the ideal air-fuel mixture ratio (Lambda = 1) for combustion.
Stereo lambda control is used on the M52 with MS42 and M54 with MS43. The oxygen sensors are mounted in the exhaust manifold.
There is only one bank on the M43 with BMS46. The oxygen sensors are mounted in the Y-pipe.
They 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. In the event of the oxygen sensor failing, the engine control unit assumes emission control with a programmed substitute value (approx. 0.45 V). This location allows the dead times for the individual exhaust paths to be reduced and more precisely monitored.
On the M52 and M54, each oxygen sensor registers three cylinders with corresponding exhaust section (cylinders 1-3 and 4-6). Test example: If the injector opening time of the first line of cylinders (cylinders 1-3) is changed, a reaction should be observed on the oxygen sensor of the first line. If this is not the case, renew the probes.
On the M43, one oxygen sensor senses all 4 cylinders.
The operation of the control oxygen sensor is similarly monitored. Malfunctions of the oxygen sensor, e.g. caused by using leaded fuel, are detected in the engine control unit by a change in the lambda control frequency.
The oxygen sensors are heated as a temperature of approx. 300 °C is required for effective operation of the oxygen sensors. The heater is actuated by the engine control unit.