Importance

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Abstract

In a system for detecting deterioration of an exhaust catalyst, when a catalyst temperature exceeds 150.degree. C., the changing widths of the output voltage of an oxygen sensor downstream of the catalyst are integrated for a predetermined sampling period to determine data .SIGMA.V reflecting the amount of purified gas component, and the deviation of the air/fuel ratio detected by an air/fuel ratio sensor upstream of the catalyst, and a target A/F and an exhaust gas flow rate or an intake flow rate are multiplied so that data .SIGMA.A/F .cndot.Q of the fluctuation of the gas component flowing into the catalyst are determined by integrating the multiplied values. At the instant when the catalyst temperature reaches 550.degree. C., the data .SIGMA.V, as integrated till then, are compared with a deterioration determining value, as set according to the data .SIGMA.A/F.cndot.Q, to determine whether the catalyst is deteriorated.

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First Claim