| 摘要 |
In an internal combustion engine, first and second oxygen sensors are installed in an exhaust system at positions upstream and downstream of a three-way catalytic converter. An air-fuel ratio control device usually controls the air-fuel ratio of mixture fed to the engine in accordance with an output issued from the first oxygen sensor. The control device is capable of correcting, upon a given diagnosing mode thereof, the air-fuel ratio of mixture to a generally stoichiometric value in accordance with a rich/lean reverse of an output issued from the second oxygen sensor. A time lag detecting device detects the elapsed time from the time on which the output from the oxygen sensor carries out a rich/lean reverse to the time on which the output from the second oxygen sensor carries out a corresponding rich/lean reverse. A judging device judges a deterioration degree of the catalyst in accordance with the time lag.
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