Abstract: When the output of the AFR ratio sensor is stable in a steady operating state, the amount of fuel is increased by step inputting to detect a point where the gradient of change in the sensor output exceeds a threshold value as a point of start of change. The time from the timing of increasing the fuel until the point of start of change is detected as a dead time. A response time is calculated until the amount of change in the sensor output reaches a predetermined ratio of the amount of change (AA?BA) of up to the steady value AA of the sensor output after the amount of the fuel is increased from the steady value BA of the sensor output before the amount of the fuel is increased. The fail condition in the AFR ratio sensor is determined based on the dead time TA and the response time TB.

Abstract: When the output of the AFR ratio sensor is stable in a steady operating state, the amount of fuel is increased by step inputting to detect a point where the gradient of change in the sensor output exceeds a threshold value as a point of start of change. The time from the timing of increasing the fuel until the point of start of change is detected as a dead time. A response time is calculated until the amount of change in the sensor output reaches a predetermined ratio of the amount of change (AA?BA) of up to the steady value AA of the sensor output after the amount of the fuel is increased from the steady value BA of the sensor output before the amount of the fuel is increased. The fail condition in the AFR ratio sensor is determined based on the dead time TA and the response time TB.