MALFUNCTION DIAGNOSING DEVICE FOR AIR/FUEL RATIO SENSOR

PROBLEM TO BE SOLVED: To improve the accuracy of diagnosing the malfunction of an air/fuel ratio sensor by distinguishing the influence of time constant deterioration of the air-fuel ratio sensor from that of waste time deterioration when diagnosing the malfunction. SOLUTION: PI control (malfunction diagnosing air-fuel ratio control) is executed to periodically change an air-fuel ratio detected by the air-fuel ratio sensor 25. A time when the air-fuel ratio detected by the air-fuel ratio sensor 25 passes through a predetermined rich-direction section during PI control is detected as a rich-direction time constant, and a time when it passes through a predetermined lean-direction section is detected as a lean-direction time constant. A time from the air-fuel ratio being corrected to be much greater to the detected air-fuel ratio exceeding a rich-direction threshold value is detected as a rich-direction waste time, and a time from the air-fuel ratio being corrected to be much smaller to the detected air-fuel ratio exceeding a lean-direction threshold value is detected as a lean-direction waste time. The rich-direction time constant, the lean-direction time constant, the rich-direction waste time and the lean-direction waste time are each compared with a malfunction determined value to diagnose the presence or not of the malfunction. COPYRIGHT: (C)2006,JPO&NCIPI