Catalyst Monitor Diagnostic Operation

Catalyst Monitor Diagnostic Operation

The catalyst monitor diagnostic measures the oxygen storage capacity of the catalyst converter. In order to do this, the heated sensors are installed before and after the three-way catalyst (TWC). Voltage variations between the sensors allow the PCM to determine the catalyst's emission performance.

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

A good catalyst has a 95 percent hydrocarbon conversion efficiency. A good catalyst shows a relatively flat output voltage on the post-catalyst HO2S signal circuit. A degraded catalyst has a 65 percent hydrocarbon conversion efficiency. A degraded catalyst shows a greatly increased activity in the output voltage from the post-catalyst HO2S.

The post-catalyst HO2S is used to measure the oxygen storage and release capacity of the catalyst. High oxygen storage capacity indicates a good catalyst. Low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S must be at the operating temperature in order to achieve reliable oxygen sensor voltages such as those shown in the three-way catalyst oxygen storage capacity illustration. The PCM performs the catalyst diagnostic during idle when the conditions for running the diagnostic are met. Refer to Conditions for Running the DTC in DTC P0420 P0420 or P0430. P0430 During the catalyst diagnostic, the PCM captures the current rear HO2S rich-to-lean status. The air/fuel ratio transitions from rich to lean or from lean to rich, depending on the initial captured rich-to-lean status. The air/fuel ratio transitions a second time opposite the first air/fuel ratio transition. During this diagnostic, the scan tool will display HO2S voltages going from full rich to full lean. This condition is normal during this diagnostic.

The catalyst monitor diagnostic is sensitive to the following conditions:

- Exhaust leaks

- HO2S contamination

- Alternative fuels such as ethanols and methanol

Exhaust system leaks may cause the following conditions:

- Prevent a degraded catalyst from failing the diagnostic

- Cause a false failure for a normally-functioning catalyst

- Prevent the diagnostic from running Some contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of any of these contaminants will reduce catalyst efficiency, and will lead to emission failures.