On-Board Diagnosis Logic

THREE WAY CATALYST FUNCTION
The Engine Control Module (ECM) monitors the switching frequency ratio of front and rear heated oxygen sensors. A three way catalyst with high oxygen storage capacity will indicate a low switching frequency of rear heated oxygen sensor. As oxygen storage capacity decreases, the rear heated oxygen sensor switching frequency will increase. When the frequency ratio of front heated oxygen sensor and rear heated oxygen sensor approaches a specified limit value, the second stage diagnosis is applied.

The second stage diagnosis switches the mixture ratio feedback control using front heated oxygen sensor to rear heated oxygen sensor. Then ECM measures the switching lag time between front heated oxygen sensor and rear heated oxygen sensor.

The longer lag time indicates the greater oxygen storage capacity. If the lag time is within the specified level, the three way catalyst malfunction is diagnosed.