Catalyst monitoring function

Description 

The catalyst stores oxygen when air-fuel mixture is lean (during air-fuel ratio correction loop control) and emits oxygen again for the HC conversion when air-fuel mixture becomes rich.

The catalyst oxygen storage capacity decreases due to aged deterioration, resulting in the decrease in conversion performance. For this reason, it is necessary to always monitor the catalyst deterioration status.

To monitor the oxygen storage capacity, ECM controls air-fuel mixture to the rich side until the value of the heated oxygen sensor reaches the maximum value. This removes most of the oxygen in the catalyst. After this, ECM controls air-fuel mixture to the lean side and measures the length of time required for the heated oxygen sensor value to reach the specified minimum value.

If the measured time is shorter than the stored specific time, ECM judges that the stored oxygen amount is insufficient and records it as an error.