Three-Way Catalyst Oxygen Storage Capacity

Three-Way Catalyst Oxygen Storage Capacity:

The TWC must be monitored for efficiency. In order to accomplish this process, the control module monitors the pre-catalyst HO2S and the post-catalyst HO2S.

When the TWC is operating properly, the post-catalyst oxygen sensor (2) will have significantly less activity than the pre-catalyst oxygen sensor (1).




The TWC stores and releases oxygen as needed during the normal reduction and oxidation process. The control module calculates the catalysts oxygen storage capacity by comparing the pre-catalyst (1) and post catalyst (2) oxygen sensors voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalysts efficiency is degraded.

The stepped or staged testing levels allow the control module to statistically filter the test information, which keeps the oxygen storage capacity test results accurate. The calculations that are performed by the on-board diagnostic system are very complex. For this reason, do not use the post-catalyst oxygen sensor activity in order to determine the oxygen storage capacity.

The efficiency of the catalyst is monitored in two stages. Failure of the first stage is an indication that the catalyst requires further testing. During the second stage, the inputs from the pre-catalyst HO2S and the post-catalyst HO2S are examined more closely in order to determine if the catalyst is indeed degraded. This additional statistical processing increases the accuracy of the oxygen storage capacity monitoring. Failing the first, 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 manufacturers sensor. These differences 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 the oxygen storage and the conversion efficiency may be altered enough to set a false DTC.