Catera V6-3.0L VIN R (1997)
Catalytic Converter: Description and Operation
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.