Description Of The Strategy

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

The diagnosis is composed of two main phases:

Measurement

Diagnosis

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.

Fig 1: Upstream Sensor Signal - Graph

Courtesy of BMW OF NORTH AMERICA, INC.

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

Diagnosis Phase

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

As an example, the typical behavior of the period criterion versus NOx emissions are shown in the following chart).

Oxygen Sensor Monitoring Diagnosis

Fig 2: Oxygen Sensor Monitoring Cycle

Courtesy of BMW OF NORTH AMERICA, INC.

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