Catalytic Converter: Description and Operation

ON BOARD DIAGNOSIS LOGIC
The ECM monitors the switching frequency ratio of front and rear heated oxygen sensors.
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 and rear heated oxygen sensors approaches a specified limit value, the three way catalyst* malfunction is diagnosed.