Three-Way Catalyst Oxygen Storage Capacity

Three-Way Catalyst Oxygen Storage Capacity:

The control module must monitor the 3-Way Catalyst (TWC) system for efficiency. In order to accomplish this, the control module monitors the pre-catalyst oxygen sensor and the post-catalyst oxygen sensor. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity (2) than the pre-catalyst oxygen sensor (1). The TWC stores oxygen as needed during the normal reduction and oxidation process. The TWC releases oxygen as needed during the normal reduction and oxidation process. The control module calculates the oxygen storage capacity using the difference between the voltage levels of the pre-catalyst oxygen sensor and the post-catalyst oxygen sensor.




If the voltage level of the post-catalyst oxygen sensor (2) approximates the voltage level of the pre-catalyst oxygen sensor (1), the catalyst efficiency is degraded.

Stepped or staged testing levels allow the control module to statistically filter test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. For this reason, do not use post catalyst oxygen sensor activity in order to determine the oxygen storage capacity unless directed by the electronic service information.

Three stages are used in order to monitor the catalyst efficiency. Failure of the first stage indicates that the catalyst requires further testing in order to determine the catalyst efficiency. Failure of the second stage indicates that the catalyst may be degraded. The third stage then monitors the inputs from both HO2 sensors more closely before determining if the catalyst is actually degraded. This further statistical processing is done in order to increase the accuracy of oxygen storage capacity type monitoring. Failing the stage 0 test or the stage 1 test does not indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content could be very high.

Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an aftermarket catalyst does not contain the same amount of cerium as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.