Catalyst Monitor Diagnostic Operation

The OBD II catalyst monitor diagnostic measures the oxygen storage capacity of the Three-way Catalytic Converter (TWC). Heated oxygen sensors are installed before (pre-catalyst) and after (post-catalyst) the TWC. Voltage variations between the sensors allow the powertrain control module to determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen is generally degraded. The OBD II catalyst monitor diagnostic is based on a correlation between the conversion efficiency of the TWC catalyst 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 HO2S.

The post-catalyst HO2S is used to measure the oxygen storage/release capacity of the catalyst in the TWC. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S 2 must be at operating temperature in order to achieve the correct oxygen sensor voltages (like those shown in the post-catalyst HO2S Outputs graphic).

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

Exhaust system leaks may cause any of the following results:
^ A false failure for a normally functioning (good) catalyst.
^ Prevent a degraded (bad) catalyst from failing the catalyst monitor diagnostic.
^ Prevent the catalyst monitor diagnostic from running.

The presence of HO2S contaminants may prevent the catalyst monitor diagnostic from functioning properly.