Theory Of Operation

General Operation:  The State of Change (SOC) catalyst monitor uses the signals from both the Upstream and Downstream Oxygen (O2) Sensors to detect aging of the catalyst. The Upstream O2 Sensor slow response monitor runs simultaneously to determine if the sensor signal is plausible. A pass/fail determination is made on the O2 Sensor slow response diagnostic before a catalyst determination is made to prevent a false fail of the catalyst 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 breakthrough the catalyst and cause the Downstream O2 Sensor to deviate from its relatively neutral baseline position to a higher state of change value. In general, the higher the Downstream O2 Sensor state of change value, the more exhaust gas breakthrough and the lower the Oxygen Storage Capacity of the Catalytic Converter. By observing and comparing the activity in the Downstream O2 Sensor signal located in the exhaust path at the Catalytic Converter, with the Upstream O2 Sensor located before the catalyst the degradation level of catalyst can be detected.

Diagnostic Algorithm:  To make the determination the Powertrain Control Module (PCM) monitors and compares the 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. So if you took the sine wave signal of both sensors over a calibrated time and straightened them out for comparison, the length of the Downstream O2 Sensor line would be approximately 20% of the length of the Upstream O2 Sensor line.