To accurately and rapidly detect the trouble of an O2 sensor situated downstream from a catalyst.
During deceleration fuel cut, during lapse of given periods (t1-t3) starting from a time after the starting of fuel cut, and when a sensor output is a rich signal, it is decided that a trouble occurs. To prevent the occurrence of erroneous detection due to consumption of oxygen for reaction to hydrocarbon occluded in a catalyst before fuel cut and discharged during fuel cut, a given period is varied according to a fuel feedback integrated time before duel cut ant by adding a fuel cut integrated time and a integrated air amount during the integrated time. For example, when the fuel feedback integrated time is longer than a threshold A, diagnosis of a trouble is quickened, and when shorter than the threshold, t1 is varied such that the diagnosis is delayed, Further, t3 is varied such that when an integrated air amount during fuel cut is much, diagnosis of a trouble is quickened and when shorter than the threshold, diagnosis of a trouble is delayed. During the cold starting, detection of a trouble may be executed.
HOSOGAI TETSUSHI
NIIMOTO KAZUHIRO
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