In order to maintain low emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the engine controls system uses a 3-way catalytic converter (TWC). The catalyst within the TWC promotes a chemical reaction, which oxidizes the HC and CO that are present in the exhaust gas. This reaction converts these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx which converts into nitrogen. The powertrain control module (PCM) monitors this process using heated oxygen sensor (HO2S) bank 1 sensor 2, which is located in the exhaust stream past the TWC. The HO2S bank 1 sensor 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to effectively convert the exhaust emissions. If the catalyst is functioning correctly, the bank 1 of the HO2S 2 signal will be far less active than the signal that is produced by bank 1 of the HO2S 1. This indicates that the TWC oxygen storage capacity is below an acceptable threshold.

The PCM performs this diagnostic test at idle. When the conditions for running this DTC are met, the following occurs:

1. The PCM captures the current rear HO2S rich-to-lean status.
2. The air-to-fuel ratio transitions from rich to lean or from lean to rich, depending on the captured rear HO2S rich-to-lean status.
3. The air-to-fuel ratio transitions a second time opposite the first air-to-fuel ratio transition.
4. The PCM captures the response time of the front and the rear HO2S when the air-to-fuel ratio transitions occur. The HO2S response time goes from less than 300 mV to more than 600 mV, and from more than 600 mV to less than 300 mV.
5. The PCM measures the time necessary for the rear HO2S voltage to cross a reference rich-to-lean threshold, minus the time necessary for the front HO2S voltage to cross the same rich-to-lean threshold. The difference between the front HO2S time and the rear HO2S time is the oxygen storage capacity of the catalyst. This DTC sets if the time exceeds a predetermined threshold.

The PCM determines that the oxygen storage capability of the TWC has degraded to less than a calibrated threshold.

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

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

5. A catalytic converter which has been discolored may be due to an engine running rich, lean or had a previous sever misfire. Verifying the fuel trim percentages may be of assistance in determining if such a condition exists.