A 3-way catalytic converter 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 present in the exhaust gas. This converts 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 PCM has the capability to monitor this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past 3-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's 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 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 deficient and the MIL will illuminate.

Then, with the vehicle stopped:

The HO2S 2 response is too quick.

The catalyst test may abort due to a change in the engine load. Do not change the engine load while a catalyst test is in progress.

An intermittent problem may be caused by the following conditions:

Thoroughly inspect any suspected circuitry for the following conditions:

Refer to Symptoms for further diagnosis.

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

2. If any component 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.

4. This step includes inspects for conditions that can cause the 3-way catalytic converter to appear degraded.

5. If you need to replace the 3-way catalytic converter, make sure that another condition is not present which could damage the converter. Correct any possible causes of converter damage before replacing the catalytic converter.

   These conditions may include the following items:

•	An engine misfire

•	A high engine oil or coolant consumption

•	A retarded spark timing

•	A weak spark