Theory Of Operation

WARNING: This page is about a different car, the 2025 Jeep Grand Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

The State of Change (SOC) catalyst monitor uses the signals from both the upstream and downstream O2 Sensors to detect aging of the catalyst. Based on the fact that when a catalyst ages, it loses some of its Oxygen Storage Capacity (OSC). As a result, part of the untreated exhaust gases can breakthrough the catalyst and causes the downstream O2 Sensor to deviate from its neutral (Stoichiometric) position. By observing the activities in the downstream O2 Sensor signal, located in the exhaust path behind the Catalytic Converter, the degradation level of catalyst can be detected. In general, the higher the downstream O2 Sensor SOC value, the more exhaust gas breakthrough and the lower the Oxygen Storage Capacity of the Catalytic Converter. The Downstream O2 Sensor response monitor is intended to diagnose a Downstream O2 Sensor that is not moving or stuck in a voltage window and to insure accurate information for catalyst monitor diagnosis.