O.sup.2 -sensor fault diagnosis method and apparatus

Abstract

An O.sub.2 -sensor fault diagnosis method and apparatus for detecting a fault of an O.sub.2 -sensor when a feedback control of a fuel supply to an engine is performed by making use of the output of an O.sub.2 -sensor and detecting and identifying a fault of the O.sub.2 -sensor even in the case where the feedback control is invalidated. A microcomputer (24) receives an output of an O.sub.2 -sensor (19) for detecting concentration of oxygen contained in an exhaust gas of an internal combustion engine (1). An ECU (20) controls a quantity of fuel supplied to the internal combustion engine (1) through feedback control in dependence on an output signal of the O.sub.2 -sensor (19). A first decision circuit is provided for deciding whether or not the O.sub.2 -sensor (19) exhibits abnormality on the basis of the output signal state of the O.sub.2 -sensor (19) by forcibly changing the amount of the fuel supplied to the engine during the feedback control, and a second decision circuit is provided for deciding abnormality of the O.sub.2 -sensor (19) in accordance with an abnormality decision process which bears a stronger correlation to the abnormality than the decision performed by the first decision circuit. Fault detection of the O.sub.2 -sensor is possible not only during the feedback control of the fuel supply but also in the state where the feedback control is not performed.