A method for investigating a cause of decrease in frequency of abnormality
detections for a certain device mounted on a vehicle, wherein when a
plurality of abnormality detection conditions are satisfied, the
abnormality detection for the certain device is performed to determine if
the certain device is in failure, the method includes (i) when the
frequency of the abnormality detections is below a predetermined value,
disabling one of the abnormality detection conditions; (ii) when the
abnormality detection conditions except the disabled abnormality
detection condition are satisfied, performing an abnormality detection
for the certain device; (iii) repeating the step (ii) a plurality of
times; and (iv) determining if the disabled abnormality detection
condition at that time is the cause of the decrease in the frequency of
the abnormality detections, based on frequency of the abnormality
detections in the step (iii).