Patent ID: 7325393
Filing Date: 2008-02-05
Classification: F01N,F02B,F02D,Y02T

Abstract:
1. A catalyst deterioration diagnosing device comprising: an exhaust gas purification catalyst provided in an exhaust passage of an engine; an upstream side sensor which detects a characteristic of an exhaust gas on an upstream side of the catalyst; a downstream side sensor which detects a characteristic of the exhaust gas on a downstream side of the catalyst; and a controller into which a detection signal of the sensors is input, functioning to: calculate a value expressing a difference in an oxygen concentration on the upstream side and the downstream side of the catalyst from the two detected values only when the engine operates at a lean air-fuel ratio; and determine that the catalyst has deteriorated when the value expressing the difference in the oxygen concentration is smaller than a deterioration determination threshold, the deterioration determination threshold being set larger as the air-fuel ratio at an inlet or an outlet of the catalyst becomes leaner, wherein both the upstream side sensor and the downstream side sensor are wide range air-fuel ratio sensors which detect the air-fuel ratio in a linear fashion.