Catalyst Monitor Diagnostic Operation

The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the 3-way catalyst (TWC). The voltage variations between the sensors allow the control module to determine the catalyst emission performance.

As a catalyst becomes less effective in promoting chemical reactions, the capacity of the catalyst to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.

A good catalyst (e.g. 95 percent hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst HO2S. A degraded catalyst (65 percent hydrocarbon conversion) shows a greatly increased activity in output voltage from the post-catalyst HO2S.

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

The catalyst monitor diagnostic is sensitive to the following conditions:
^ Any exhaust leaks
^ Any contamination of the HO2S 2
^ Any alternative 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 following contaminants that may be encountered:
^ Phosphorus
^ Lead
^ Silica
^ Sulfur

The presence of these contaminants prevents the TWC diagnostic from functioning properly.