2003 3BMXV03.0UL2
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- General Description (Monitoring Procedure) - Catalyst monitoring is based on oxygen storage capacity. Engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in exhaust gas. These oscillations are dampened by oxygen storage activity of catalyst. Amplitude of remaining lambda oscillations downstream of catalyst indicates storage capability. To determine catalyst efficiency, model-based nominal amplitude of upstream oxygen sensor is modified to predefined higher values (forced stimulated). This results in a known area of A/F-controller, whereby magnitude of oxygen admission to catalyst is equal, independent of driving conditions. See Figure.
- Magnitude of oxygen admission is used to calculate maximum permissible oscillation (areas of cycle) of downstream sensor of a properly functioning catalyst. A fixed number of complete lambda controller cycles are used to calculate areas which are enclosed by voltage of downstream sensor and also to calculate mean value. See Figure. Now original measured oscillation (average of areas) from downstream sensor is compared to calculated maximum permissible value. A fault is detected if quotient (measured value to calculated value) is greater than threshold. See Figure.