| 摘要 |
<p>PURPOSE: To perform high-precise determination of a three-dimensional catalyst even when an output signal from a downstream side O2 sensor is shifted to the rich side or the lean side. CONSTITUTION: After an ECU 40 calculates an average value VORave based on the output voltage VOR of a downstream side O2 sensor 15, a hysteresis constantαis added thereto and subtracted therefrom to calculate upper and lower inversion reference values THH and THL. Thereafter, after a downstream inversion frequency fR is determined from the number of times which an output voltage VOR crosses the inversion reference values THH and THL, the ECU 40 calculates an inversion frequency ratio fR/fF from the output voltage VOF of an upper stream side O2 sensor 14 determined by the number of times which the output voltage VOF of an upper stream side O2 sensor 14 crosses a threshold value VTH. Thereafter, the ECU 40 diagnoses deterioration of a three- dimensional catalyst 13 when a value of the inversion frequency ratio fR/fF exceeds a given value E1, diagnoses that a purification efficiency ECAT is decreased to a value lower than a lower limit value E1 and lights ON an alarm lamp 41, and stores a trouble cord at an RAM.</p> |
