Catalyst Monitor Diagnostic Operation

The OBD II catalyst monitor diagnostic measures the oxygen storage capacity of the catalyst. In order for this process to happen, the heated sensors are installed before and after the Three-Way Catalyst (TWC). The voltage variations between the sensors allow the control module to determine the catalyst's emission performance. As a catalyst becomes less effective in promoting any chemical reactions, the catalyst's capacity to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on a correlation between the conversion efficiency and the oxygen storage capacity of the catalyst. A good catalyst (e.g. 95 percent hydrocarbon conversion efficiency) will show a relatively flat output voltage on the post-catalyst heated oxygen sensor (HO2S 2). A degraded catalyst (65 percent hydrocarbon conversion) will show greatly increased activity in the output voltage from the post catalyst heated oxygen sensor (HO2S 2).

The post-catalyst HO2S measures the oxygen storage/release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst, and a low oxygen storage capacity indicates a failing catalyst. In order to achieve accurate oxygen sensor voltage readings like those shown in the Post-Catalyst HO2S Outputs graphic, the TWC and the HO2S 2 must be at the normal operating temperature.

The catalyst monitor diagnostic is sensitive to the following conditions:
^ Exhaust leaks
^ HO2S contamination
^ Alternate fuels

Exhaust system leaks may cause the following results:
^ Prevent a degraded catalyst from failing the diagnostic
^ Cause a false failure for a normally functioning catalyst
^ Prevent the diagnostic from running

Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants at any HO2S will prevent the TWC diagnostic from functioning properly.