PURPOSE: To judge easily even a failure condition with minor reflection upon a change in an air-fuel ratio itself such as clogging of a fuel injection valve in a particular cylinder by judging a failure in the fuel system of a part of cylinders based on the turbulence of an outputted waveform from an air-fuel ratio sensor detected by a waveform detection means which detects the waveform of an outputted signal from the air-fuel ratio sensor.
CONSTITUTION: Upstream of a three way catalytic converter 11 in an exhaust tube 10, an O2 sensor 16 is provided, for detecting oxygen concentration (A/F) in the exhaust gas which flows in an exhaust passage 10a. An engine controller (ECU) 9 compares the output of the O2 sensor 16 with a fixed reference voltage value corresponding to a stoichiometric air-fuel ratio λ=1, and increases or decreases supplied fuel amount for controlling an actual air-fuel ratio so as to keep it around the stoichiometric air-fuel ratio. In the ECU9, an engine data signal necessary for control is stored. Moreover, a change cycle which crosses the slice level of an outputted signal waveform from O2 sensor 16 is stored, and is compared with the change cycle of a reference value which indicates an abnormal condition so as to judge a failure.
TERADA KOICHI
TOKUDA YOSHIHARU
