Importance

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Abstract

The device according to the invention detects deterioration of a catalytic converter disposed in an exhaust passage of an internal combustion engine based on output signal response curves of the air-fuel ratio sensors disposed in the exhaust passage upstream and downstream of the catalytic converter, by determining that the catalytic converter has deteriorated when relationship of a ratio of the lengths of the output response curves of the upstream and downstream air-fuel ratio sensors and a ratio of the areas bounded by the output signal response curves and reference lines satisfies predetermined conditions during air-fuel ratio feedback control of the engine; to eliminate errors the detection during a transition of the operating condition of the engine, the device also determine a degree of transition of the operating condition of the engine, for example, by detecting number of reversals of the output signal of the upstream air-fuel ratio sensor, and corrects the value of the ratio of the areas of the output signal response curves in accordance with the degree of the transition of the operating condition of the engine; thus the deterioration of the catalytic converter is detected accurately both in the transition operating condition and in the stable operating condition.

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First Claim