Description Of The Strategy

O2 sensor monitoring is a sequential diagnosis made on steady state conditions.

The diagnosis is composed of two main phases: Measurement Diagnosis and Measurement Phase.

The algorithm is based on the period measurement (starting from lean to rich sensor transition). To avoid non-representative measurement, the period is valid only if the sensor has been below a low threshold and above a high threshold between 2 consecutive lean/rich transitions.

If one of the diagnostic conditions is not met, the test is stopped and the system returns to the OUT OF DIAGNOSIS state.

Fig 1: Oxygen Sensor Monitoring Measurement Phase

Courtesy of BMW OF NORTH AMERICA, INC.

Diagnosis Phase

The sum of the periods is compared to limits values. to declare a failure.

Typical behavior of the period criterion versus NOx emissions for instance.

If O2 sensor diagnosis conditions are fulfilled, the system informs the general OBD sequencer and waits for its authorization to start measurement phase. The OBD sequencer manages the priorities in case of multiple diagnosis requests (catalyst diagnosis and O2 sensor diagnosis).

Fig 2: Oxygen Sensor Monitoring Diagnosis

Courtesy of BMW OF NORTH AMERICA, INC.