| 摘要 |
PURPOSE:To protect an engine, by a method wherein, in a device which calculates an air-fuel ratio according to a control current fed between an oxygen sensor and a pump cell, during control to other than a theoretical air-fuel ratio, abnormality is decided based on inclination calculated from the calculated air-fuel ratio and a set value. CONSTITUTION:An integration amplifier 11 integrates a signal from a comparing circuit 10 through which to interconnect a reference electrode 8 in a pump cell 1 and a reference source equivalent to an electromotive force available during combustion in theoretical air-fuel ratio, aud an electric control signal is outputted a pump electrode 3A. The control current is outputted as an air-fuel ratio calculating signal through a voltage detecting circuit 12 and on adding circuit 13, the direction of the control current is inputted to a second detecting means DM2, and a theoretical air-fuel ratio is decided. A controller 19 calculates inclination based on on air-fuel ratio calculating signal and a preset value, and when the inclination exceeds an allowable margin, it is decided that abnormality occurs, and shifting to an open loop mode is effected.
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