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 break through the catalyst causing the Downstream O2 Sensor to deviate from its relatively neutral baseline position to a higher State of Change  value. In general, as more exhaust gases break through and lower the Oxygen Storage Capacity of the Catalytic Converter, the Downstream O2 Sensor State of Change  will increase and get closer to the Upstream O2 Sensor State of Change  . 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.

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. 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.