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 Three-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, which operates at approximately 95 percent hydrocarbon conversion efficiency, shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S). A degraded catalyst, which may operate at approximately 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, the HO2S 2, and the HO2S 3 must be at operating temperature in order to achieve correct oxygen sensor voltages like those shown in the post-catalyst HO2S 3 outputs graphic.

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

Exhaust system leaks can lead to 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 prevents the TWC diagnostic from functioning properly.