Catalyst Monitoring: Description
Catalyst monitoring is based on the monitoring of oxygen storage capability. The engine closed loop feedback control generates Lambda (air fuel ratio) oscillations in the exhaust gas. These oscillations are damped by the oxygen storage activity of the catalyst. The amplitude of the remaining Lambda oscillations downstream of the catalyst indicates the storage capability.
In order to determine catalyst efficiency, the amplitude ratio of the signal oscillations of the upstream and downstream Lambda sensors is determined. 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 catalyst is diagnosed.
NOTE:
Definition of Lambda: The stoichiometric air fuel ratio is the mass ratio of 14.7 kg of air to 1 kg of gasoline theoretically necessary for complete combustion. The excess air ratio (Lambda) indicates the deviation of the actual air fuel ratio from the theoretical air fuel ratio. Thus Lambda = actual inducted air mass/theoretical air requirement.