Catalyst Monitor Diagnostic Operation: Description

Catalyst System Low Efficiency diagnostic monitors the operation of the exhaust system catalytic converter. In order to control exhaust emission of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx), a three-way catalyst converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx into nitrogen. The Powertrain Control Module (PCM) has the capability to monitor this process using a heated oxygen sensor (HO2S) located in the exhaust stream past the three-way catalytic converter. The PCM compares the signal from the heated Oxygen Sensor 1 (HO2S 1), located before the catalyst with the Sensor 2 (HO2S 2) located after the catalyst in order to determine if the catalyst has deteriorated. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than the HO2S 1. Both HO2S signals changing at similar rates indicates that the catalyst is deteriorating.

Voltage variations between the sensors allow the PCM to determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen is generally degraded. The OBD-II catalyst monitor diagnostic is based on a correlation between the conversion efficiency of the TWC catalyst and the oxygen storage capacity of the catalyst. A good catalyst, 95 percent hydrocarbon conversion efficiency, will show a relatively flat output voltage on the post-catalyst sensor, HO2S 2. A degraded catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post catalyst HO2S 2. See Figure and Figure .

The post-catalyst HO2S is used to measure the oxygen storage and release capacity of the catalyst in the TWC. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S 2 must be at operating temperature in order to achieve the 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 any of the following results:

• A false failure for a normally functioning, good catalyst.
• Prevent a degraded catalyst from failing the catalyst monitor diagnostic.
• Prevent the catalyst monitor diagnostic from running.