In order to maintain a reasonably low emission level of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and CO present in the exhaust gas. This reaction converts the gases into harmless water vapor and carbon monoxide. The catalyst also reduces the NOx , converting the NOx, into nitrogen. The engine control module (ECM) monitors this process using the post catalyst heated oxygen sensor (HO2S) signal. The post catalyst HO2S, located in the exhaust stream after the three-way catalytic converter, produces an output signal that indicates the oxygen storage capacity of the catalyst. The catalyst's oxygen storage capacity determines the catalyst's ability to convert the exhaust emissions effectively. If the catalyst is functioning correctly, the post catalyst HO2S signal will be far less active than the signal produced by the pre-catalyst HO2S. If the ECM detects an excessive post catalyst signal activity (outside of an acceptable range) for an extended period of time, the ECM will set this DTC. This indicates that the three-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable.

The ECM illuminates the malfunction indicator lamp (MIL) and records the operating conditions in the Freeze Frame on the second consecutive drive trip that the diagnostic runs and fails.

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

4. Before replacing the three way catalytic (TWC) converter, correct any conditions which may have caused the catalyst to be damaged.