A 3-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 process will convert the HC and the CO into a harmless water vapor and CO, and will reduce 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 3-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, 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 malfunction indicator lamp (MIL) will illuminate.

With the vehicle stopped:

The PCM has determined the catalyst efficiency has degraded below a calibrated threshold.

The catalyst test may abort due to a change in the engine load. Do not change the engine load, that is, do not reset the A/C, the coolant fan, or the heater motor while a catalyst test is in progress.

Certain conditions may cause a catalytic converter to degrade. These conditions may include the following conditions:

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.

3. This step includes checks for conditions that can cause the 3-way catalytic converter to appear degraded. Before you proceed with this table, repair any conditions that you find.

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