A three-way catalytic converter (TWC) is used in order to control the exhaust emissions of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that is present in the exhaust gas. This will convert the HC and the CO into a harmless water vapor and a carbon dioxide, and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also has the ability to store oxygen. The powertrain control module (PCM) has the capability to monitor this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past the TWC. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. The PCM monitors the catalyst efficiency by first allowing the catalyst to heat up, waiting for a stabilization period while the engine is idling. Then, the PCM adds and removes the fuel while monitoring the reaction of the HO2S. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the oxygen sensor (O2S 1). When the HO2S 2 response is close to that of the O2S 1, the oxygen storage capability or efficiency of the catalyst is considered to be bad and the malfunction indicator lamp (MIL) will illuminate.

The PCM determines that the oxygen storage capacity of the TWC is degraded past a calibrated value.

Thoroughly check any circuitry suspected of causing the intermittent complaint . Inspect for the following conditions :

The numbers below refer to the step numbers on the diagnostic table.

2. If any component diagnostic trouble codes (DTCs) are set, diagnose those DTCs first. A malfunction in a component can cause the converter to appear degraded or may have caused the malfunction.

3. This step includes inspecting for conditions that can cause the TWC to appear degraded. Repair any problems found before proceeding with this table.

4. If you need to replace the TWC, make sure that another condition is not present which would cause damage to the converter. Correct any possible causes of converter damage before replacing the TWC in order to avoid damage to the replacement converter.