2003 3BMXV03.2S54 & 3BMXV04.9S62
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- Catalyst Monitoring General Description - Catalyst monitoring is based on monitoring of oxygen storage capability. 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, oscillation of upstream sensor is needed to calculate oxygen input and output (catalyst) by engine air mass and lambda-deviation. Downstream sensor signal for a threshold catalyst is then derived from this basic value. Anytime real sensor signal oscillation (downstream) corresponds to model, a defective catalyst is recognized. See Fig 1.
- Computation Of Efficiency Of Catalyst - Alternating current component of voltage of oxygen sensors before and after catalyst is determined, rectified and averaged. Actual quotient of oxygen sensor voltage before catalyst and oxygen sensor voltage after catalyst is determined. Simultaneously, theoretical quotient of these voltages is computed in dependency of operating point of engine. Respective operating point is determined using parameters load and engine speed.
- Fault Evaluation - At end of diagnostic period, number of stored values in adaptation matrix exceeding a limit is determined. If this number of stored values itself exceeds a threshold, a defective catalyst is evaluated.
- Check Of Monitoring Conditions - Monitoring principle is based on detection of relevant oscillations of downstream sensor signal during regular lambda control. It is necessary to check driving conditions for exceptions where no regular lambda control is possible. During such periods, and for a certain time afterward, computations of amplitude values and subsequent processing is interrupted so a distortion of monitoring information is avoided.