Catalytic converter diagnosis

The catalytic converter diagnosis function tests the ability of the catalytic converter to store oxygen. The oxygen-storage capacity serves as an index of the catalyst's ability to perform chemical conversion.

During the first phase of the catalyst diagnosis process (lasting roughly 3 seconds) the system dials in a rich mixture and maintains it until the oxygen sensor's voltage reaches a predefined value. Because rich exhaust is low in oxygen, there is a corresponding reduction in the amount of oxygen stored in the catalytic converter.

In the 2nd phase a lean mixture with oxygen-rich exhaust gas is supplied. The catalytic converter's ability to store oxygen is calculated based on the time that elapses before the post-catalyst oxygen sensor reacts to the adjustment in exhaust-gas composition. The operative assumption is that no change in the oxygen sensor's voltage occurs until the catalytic converter reaches the limit of its ability to store oxygen.

The system recognizes the catalytic converter as defective when its ability to store oxygen falls below a specified threshold value.