Exhaust purification system for multiple cylinder engines

摘要

An exhaust purification system has an upstream exhaust gas sensor and a downstream exhaust gas sensor disposed downstream of a catalytic device. Each sensor detects an oxygen concentration in exhaust gases and provides an output which inverts between a lean state, representative of a lean air/fuel ratio, and a rich state, representative of a rich air/fuel ratio, according to oxygen concentrations. The upstream exhaust sensor is determined to have deteriorated when an output from the upstream exhaust sensor is kept in a predetermined correlation with an output from the downstream exhaust sensor while the output from the downstream exhaust sensor remains the same.