Catalyst Monitoring System: Notes
Upstream O2 Sensor Monitoring
The Upstream O2 Sensor State of Change (SOC) monitor is used to detect a failed Upstream O2 Sensor. The Upstream O2 Sensor is very important for emission control and engine performance since it can provide immediate air/fuel ratio feedback. Therefore, proper operation of this Upstream O2 Sensor is critical. An aged Upstream O2 Sensor tends to be less active and has a slower response rate to changes in the air/fuel ratio of exhaust gas. As a result, the quality of the Upstream O2 Sensor can be determined by observing the voltage activity from the sensor. This monitor is triggered when the engine is running at certain MAP/RPM windows, airflow is relatively stable and steady O2 control is applied. The SOC for Upstream O2 signal is captured in an internal calibrated time block (consisting of consecutive samples), such that even the slowest (but still considered as passing) O2 Sensor should accumulate enough state changes to pass the monitor.
Catalyst Failure Rationality
The catalyst determination is based on the fact that when a catalyst ages it loses some of its Oxygen Storage Capacity (OSC). As a result, the exhaust gases can break through the catalyst causing the Downstream O2 Sensor to deviate from its relatively neutral baseline position to a higher SOC value. In general, as more exhaust gases break through and lower the Oxygen Storage Capacity of the Catalytic Converter, the Downstream O2 Sensor SOC will increase and get closer to the Upstream O2 Sensor SOC . By observing and comparing the activity in the Downstream O2 Sensor signal, located in the exhaust path downstream of the Catalytic Converter, with the Upstream O2 Sensor located before the catalyst the degradation level of catalyst can be detected.
There are several different catalyst efficiency monitoring windows. Each specific window has several different enable conditions. The best opportunity for the catalyst efficiency monitor to run is:
- Engine temperature above 70°C (158°F)
- Engine speed approximately 1000-3000 RPM
- Vehicle speed 32-137 km/h (20-85 mph), under moderate engine load
Catalyst System Diagnostic Monitor
To make the determination, the Powertrain Control Module (PCM) monitors and compares the SOC ratio between the Downstream O2 Sensor and the Upstream O2 Sensor over a calibrated period of time. Typically, if the catalyst is working properly, the ratio between the Downstream O2 Sensor and the Upstream O2 Sensor is approximately (0.2/1). A simple way to understand this comparison is to understand that the shortest distance between two points is a straight line. Using this logic, if the sine wave signal of both sensors was recorded for a calibrated time period and straightened out for comparison, the length of the Downstream O2 Sensor line would only measure approximately 20% of the length of the Upstream O2 Sensor line. As the Catalytic Converter begins failing, the ratio will move towards 1/1. Typically, if the ratio becomes greater than a calibrated threshold, approximately 0.6/1, the PCM will consider the catalyst as failing and fault the system.