Every petrol engine always requires an exactly defined air/fuel ratio in order to comply with the prescribed emission values. In order to maintain the necessary optimised efficiency of the catalytic converters, the aim is to achieve the ideal air/fuel ratio (Lambda = 0.9 to 1.1).
Lambda oxygen sensors continuously compare the residual oxygen in the emissions and forward the corresponding voltage values to the control unit. There, this signal can be used to continuously ascertain whether the combustion runs with a rich (Lambda less than 1.0) or lean mixture (Lambda greater than 1.0). If a correction is necessary, this is achieved by varying the fuel injection timing.
In the event of a Lambda oxygen sensor failing, the engine control unit assumes emission control with a programmed substitute value. This arrangement shortens the dead times of the individual exhaust gas routes and sensing is more exact.
The Lambda oxygen sensor on the S54 engine is a continuous measuring sensor (wideband probe) fitted in the exhaust line in the pre-catalytic converter. Furthermore, a ”monitor” sensor in the post-catalytic converter monitors the correct function of the catalytic converter. This is a jump sensor (heated). In the event of a failure, a fault code memory entry is made and the driver is notified by means of the warning lamp.
Failures of the Lambda oxygen sensors, e.g. caused by using leaded fuel, are detected in the engine control unit by a change in the regulating oxygen sensor's frequency.
On the S54, each pair of sensors picks up three cylinders with corresponding exhaust lines (cylinders 1-3 and 4-6). Test example: If the opening times of the fuel injectors of the first row (cylinders 1-3) are changed, there must also be a reaction at the Lambda oxygen sensor of the first row. If this is not the case, renew the probes. There is a separate test module to check for swapped Lambda oxygen sensors.
During a visual inspection, bear the following in mind:
Heavy rust deposits lead to clogging of the sensor and diminish the reaction time. The mixture can then be too rich or the sensor heater can be damaged. Heavy white or grey deposits indicate that fuel additives are being used or the engine is burning oil.