| 摘要 |
PROBLEM TO BE SOLVED: To detect abnormality of an oxygen sensor by using a frequency characteristic of an electric current and voltage of an oxygen concentration sensor. SOLUTION: An oxygen sensor 5 has a detecting element part 6 to output an almost constant limiting current corresponding to the oxygen concentration (the air-fuel ratio) according to the impression of voltage and a heater 7 to heat the detecting element part 6. A CPU 2a in a microcomputer 2 detects an internal resistance of the oxygen sensor 5, and controls current-carrying of the heater 7 in a feedback process so that its internal resistance coincides with a desired value. The CPU 2a switches impression voltage to prescribed voltage to detect abnormality, and analyzes a frequency characteristic of an electric current change according to switching of its voltage. A cutoff frequency fc being an inflection point of a gain characteristic of a frequency response is used as an abnormality judging parameter, and when its parameter is not in a prescribed normal area, the fact that the sensor is abnormal is judged. |
