| 摘要 |
PURPOSE:To enable prevention of the occurrence of erroneous diagnosis when an exhaust gas concentration sensor has low temperature, according to the method wherein, during low load running including idle running of an engine, an exhaust gas concentration detecting system is prevented from detection of a trouble. CONSTITUTION:In an electronic control unit 5, an upper limit discriminating value and a lower limit discriminating value are set within a range set by the upper and lower limit values of an air-fuel ratio correction value determined during normal running of an engine 1. When an air-fuel correction value continues a value outside a range set by the upper and the lower discriminating values for a given time, an exhaust gas concentration detecting system including an exhaust gas concentration sensor 15 is decided to has a trouble. In which case, it is discriminated whether the engine 1 is in an idle running condition or not, and when the result is YES, since the temperature of the exhaust gas concentrationi sensor 15 is low and its activation is insufficient, an output voltage becomes unstable and causes erroneous diagnosis to occur to detection of a trouble, and thereby the system is prevented from detection of a trouble.
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