System Description

The ECM has the capability of monitoring three way catalytic converter efficiently by using a second Heated Oxygen Sensor (Rear HO2S) located in the exhaust, downstream of the TWC converter. This sensor will also produce an output signal indicating the oxygen storage capacity of the TWC converter which will indicate how effectively the TWC can convert exhaust emissions into harmless gases.
TWC monitoring is based on monitoring the oxygen storage capability. The correction between conversion efficiency and storage capability has been shown in various investigations. The engine control results in regular lambda oscillations of the exhaust gas (lambda = normalized air fuel ratio). These oscillations are dampened by the storage activity of the three way catalytic converter.
The amplitude of the remaining lambda oscillations downstream of the TWC converter indicates its storage capability. The method uses the amplitude ratio of the signal oscillations of upstream and downstream lambda sensors as raw information. This information is evaluated separately in different engine load and speed ranges. If there is an indication of low storage capability in a certain number of operating ranges, a defective TWC converter is recognized. According to the described operating principle, the following main parts can be distinguished:
- Computation of the amplitude ratio as raw information
- Post processing in different engine load and speed rages
- Fault evaluation
- Check of monitoring conditions