Three-Way Catalyst Oxygen Storage Capacity

Oxygen Sensors And Catalytic Converters:

Catalyst Monitoring

The Three-Way Catalyst (TWC) must be monitored for efficiency. In order to accomplish this process, the control module monitors the pre-catalyst Heated Oxygen Sensor (HO2S) and the post-catalyst HO2S oxygen sensors. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity than the pre-catalyst oxygen sensor.

Oxygen Storeage

The TWC stores and releases the oxygen as needed during the normal reduction and oxidation process. The control module will calculate the oxygen storage capacity by using the difference between the pre-catalyst and post catalyst oxygen sensor's voltage levels. If the activity of the post catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst's efficiency is degraded.

Testing Strategies

The stepped or staged testing levels allow the control module to statistically filter the test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. 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 unless you are directed.

Catalyst Efficiency

Two stages are used in order to monitor the efficiency of the catalyst. Failure of the first stage will indicate that the catalyst requires further testing in order to determine the efficiency of the catalyst. The second stage then looks at the inputs from the pre catalyst HO2S sensors and the post catalyst HO2S sensors more closely before determining if the catalyst is indeed degraded. This further statistical processing is done in order to increase the accuracy of the oxygen storage capacity type 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 Components

The aftermarket HO2S characteristics may be different from the original equipment manufacturer's sensor. These characteristics 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.