AIR FUEL RATIO CONTROL DEVICE

摘要

PROBLEM TO BE SOLVED: To provide an air fuel ratio control device capable of detecting a trouble when an output characteristic change affecting the purging rate of a catalyst is caused in an oxygen concentration sensor. SOLUTION: An O2 sensor 18 is provided in the downstream of a three-way catalyst 19 and an LAF sensor 17 is provided in the upstream thereof in an exhaust system of an engine 1. In controlling the air fuel ratio according to the outputs of the sensors 18, 17, an ECU 5 can detect a trouble in the LAF sensor 17 in a stoichiometric region by determining the stoichiometric deterioration of the LAF sensor 17 when the average detection equivalance ratio KACTAV corresponding to the output average value of the LAF sensor 17 (upstream oxygen concentration sensor) when the target air fuel ratio is controlled according to the output of the O2 sensor 18 (downstream oxygen concentration sensor) exceeds a designated range.