Three-Way Catalyst Oxygen Storage Capacity

The TWC catalyst must be monitored for efficiency. In order to accomplish this, the control module monitors the pre-catalyst HO2S and post-catalyst HO2S. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity than the pre-catalyst oxygen sensor. The TWC stores and releases oxygen as needed during its normal reduction and oxidation process. The control module will calculate the oxygen storage capacity using the difference between the pre catalyst and post catalyst oxygen sensors voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst efficiency is degraded.

Stepped or staged testing levels allow the PCM to statistically filter test information. This prevents falsely passing or falsely failing the catalyst monitor oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. Post-catalyst oxygen sensor activity should not be used to determine oxygen storage capacity unless directed by the service manual.

A two stage test is used to monitor the catalyst efficiency. Failure of the first stage of the test will indicate that the catalyst requires further testing in order to determine the catalyst efficiency. The second stage test looks at the inputs from the pre-catalyst and post-catalyst HO2S more closely in order to determine if the catalyst is actually degraded. This two stage test further increases the accuracy of the oxygen storage capacity monitor. Failing the first stage test DOES NOT indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content may be very high.

After-market HO2S characteristics may be significantly different from the original equipment manufacturer HO2S. An inferior HO2S may lead to a false pass or a false fail of the catalyst monitor diagnostic. An after-market catalytic converter that does not contain the same amount of cerium as the original catalytic converter can cause a false DTC to set. An incorrect amount of cerium in the catalyst can alter the correlation between the oxygen storage and the conversion efficiency of the TWC.