Catalyst Monitor Diagnostic Operation

The catalyst monitor diagnostic measures oxygen storage capacity of the catalyst converter. In order to do this, the HO2S are installed before and after the TWC. Voltage variations between the sensors allow the PCM to determine the catalyst emission performance.

As a catalyst becomes less effective in promoting chemical reactions, the catalysts capacity to store and release oxygen generally degrades. The 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 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 the HO2S must be at Operating temperature in order to achieve 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
^ Alternative fuels

Exhaust system leaks may cause the following:
^ 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 prevent the TWC diagnostic from functioning properly.