Abstract: A valve operation verifying system for making verification of valve operation of an EGR valve of a type having a diaphragm elastically deformable according to operating conditions of an engine to admit controlled amounts of exhaust gas that is recirculated into an intake air stream which verifies, on a basis of gas pressure in an EGR passage, whether the diaphragm remains elastically deformable as specified and whether a pressure sensor remains free from freezing, and executes the verification of valve operation of the EGR valve on a basis of a change in the gas pressure in the EGR passage when the EGR valve opens and when it closes, and when the temperature of ambient air is lower than a critical point, verifying that the diaphragm remains elastically deformable as specified and that said pressure sensor is free from freezing.

Abstract: A diagnosis of deterioration and/or normality of an exhaust gas purifying catalyst is made by comparing a ratio of reversal frequencies of outputs from oxygen sensors disposed on opposite sides of the exhaust gas purifying catalyst with a threshold ratio. Discrimination is performed on both normality and deterioration in a standard gas quantity zone, on only normality in a large gas quantity zone, and on only deterioration in a small gas quantity zone, and nothing is made in an extra large gas quantity zone.

Abstract: Detection of abnormalities of an idle speed control system is performed based on whether a deviation of an on-idle engine speed from a target engine speed is within a critical range which is changed so as to make the idle speed control system harder to be judged abnormal when the engine is in a lapped state where the engine is apt to change speed than when in a green state where the engine is hardly changeable in speed.