Three-Way Catalyst Oxygen Storage Capacity

The PCM must monitor the Three-way Catalyst System (TWC) for efficiency. In order to accomplish this, the PCM monitors the pre-catalyst and post-catalyst oxygen sensors. When the TWC is operating properly, the post-catalyst (2) oxygen sensor will have significantly less activity than the pre-catalyst (1) oxygen sensor. The TWC stores oxygen during its normal reduction and oxidation process. The TWC releases oxygen during its normal reduction and oxidation process. The PCM calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor's voltage levels.




Whenever the sensor activity of the post-catalyst oxygen sensor (2) nears the sensor activity of the pre-catalyst oxygen sensor (1), the catalyst's efficiency is degraded.

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