In order to maintain a reasonably low emission level of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) a 3-way catalytic converter (TWC) 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 dioxide. 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 TWC, produces an output signal that indicates the oxygen storage capacity of the catalyst. The oxygen storage capacity determines the ability of the catalyst 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 diagnostic trouble code (DTC). This indicates that the TWC oxygen storage capacity is below a threshold considered acceptable.

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