Catalyst Monitor Diagnostic Operation

The catalyst monitor diagnostic measures 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 emission performance. As a catalyst becomes less effective in promoting chemical reactions, the catalyst's 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% hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S) signal circuit. A degraded catalyst (65% 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 reliable oxygen sensor voltages like those shown in the Three-Way Catalyst Oxygen Storage Capacity graphic.

The PCM performs the catalyst diagnostic at idle when the conditions for running the diagnostic are met. Refer to Conditions for Running the DTC in DTC P0420 Catalyst System Low Efficiency Bank 1 or DTC P0430 Catalyst System Low Efficiency Bank 2m. During the catalyst diagnostic the PCM captures the current rear HO2S rich/lean status. The Air Fuel ratio transitions from rich to lean or lean to rich depending on the initial captured rich/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 (Ethanols and Methanols)

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 any of these contaminants will reduce catalyst efficiency and lead to emission failures.