Oxygen Sensor Monitoring
Oxygen Sensor Monitoring
General Overview
The upstream sensor will cause emission increase when its response time increases too much (A/F Loop period or frequency check).
The period of the A/F loop is measured and the number of lean/rich transition are counted. The sum of valid periods is then calculated.
The corresponding limit period versus operating point (N, MAF) is acquired.
A failure is declared when:
The sum of the measured periods exceeds the sum of the corresponding limit.
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
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 <
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 (see following chart).
Oxygen Sensor Monitoring Diagnosis
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).