Monitor Description

The ECM uses sensors mounted in front of and behind the Three-way Catalytic Converter (TWC) to monitor its efficiency.

The first sensor, the air fuel ratio sensor, sends pre-catalyst information to the ECM. The second sensor, the heated oxygen sensor, sends post-catalyst information to the ECM.

In order to detect any deterioration in the Three-way Catalytic Converter (TWC), the ECM calculates the Oxygen Storage Capacity (OSC) of the Three-way Catalytic Converter (TWC). This calculation is based on the voltage output of the heated oxygen sensor while performing active air-fuel ratio control.

The OSC value is an indication of the oxygen storage capacity of the Three-way Catalytic Converter (TWC). When the vehicle is being driven with a warm engine, active air-fuel ratio control is performed for approximately 15 to 20 seconds. When it is performed, the ECM deliberately sets the air-fuel ratio to lean or rich levels. If the rich-lean cycle of the heated oxygen sensor is long, the OSC becomes greater. There is a direct correlation between the OSCs of the heated oxygen sensor and Three-way Catalytic Converter (TWC).

The ECM uses the OSC value to determine the state of the Three-way Catalytic Converter (TWC). If any deterioration has occurred, it illuminates the MIL and stores the DTC.