General description of Catalyst Monitoring

Catalyst monitoring is based on oxygen storage capacity.

The engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in the exhaust gas. These oscillations are dampened by the oxygen storage activity of the catalyst. The amplitude of the remaining lambda oscillations downstream of the catalyst indicates the storage capacity.

In order to determine catalyst efficiency the oscillation of the upstream sensor is used to calculate the oxygen input and output of the catalyst "based on engine air mass and lambda-deviation. The downstream sensor signal for a "threshold catalyst" then is derived (modeled) from this basic value.

Any time the actual downstream sensor signal oscillation corresponds to the model, a defective catalyst is recognized.