A three-way catalytic (TWC) 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 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 three-way catalytic converter. 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 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 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 (i.e. the A/C, the coolant fan, the heater motor) while a catalyst test is in progress.

An Intermittent problem may be caused by the following conditions:

Thoroughly check any suspected circuitry for the following conditions:

Refer to Symptoms for further diagnosis.

The number(s) below refer(s) to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD System Check prompts you to complete some of the basic checks and to store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The scan tool stores this data for later reference.

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.

3. This step includes checks for conditions that can cause the three-way catalytic converter to appear degraded. Repair any problems found before proceeding with this table.

5. If you need to replace the three-way catalytic converter, 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 catalytic converter in order to avoid damage to the replacement converter.

   These conditions may include the following items:

•	Engine misfire

•	High engine oil or coolant consumption

•	Retarded spark timing

•	Weak spark

6. Clearing the DTCs allows the catalyst test to be run up to 6 times this ignition cycle. Once the ignition switch is cycled, the test will run only once. Driving the vehicle heats the catalyst to a test temperature. The engine speed must be above idle for a predetermined amount of time before the PCM will allow the catalyst test to run. Once at idle, the PCM will allow the system to stabilize and then test the catalyst in 2 stages.

7. If you have found no malfunctions at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.