| 摘要 |
<p>PURPOSE: To restrain the effect on workability by performing diagnosis by a second deterioration diagnostic means only when the judgement of deterioration can not be performed by a first deterioration diagnostic means by the comparison of output signals of upstream and downstream air-fuel ratio sensors. CONSTITUTION: When the condition that a catalyst is to be diagnosed in the air-fuel ratio feedback control state is judged, diagnosis is performed by a first deterioration diagnostic means on the basis of the rate of the rich and lean inversion periods of output signals of first and second O2 sensors 8, 9 on the upstream and downstream sides. When the judgement of deterioration can not be performed by the first deterioration diagnosis by the comparison of the rate with the specified value, diagnosis is performed by a second diagnostic means. That is, the air-fuel ratio of engine intake mixture is forcibly made lean, and this state is detected by the first and second O2 sensors 8, 9 is confirmed, after that, the air-fuel ratio is inverted into the rich state. The delay time from the tine when the rich inversion is detected by the first O2 sensor 8 to the time when the rich inversion is detected by the second O2 sensor 9 is measured and compared with the specified value, and the deterioration is diagnosed.</p> |
