| 摘要 |
<p>An exhaust gas purifying apparatus for an internal-combustion engine, including a three-way catalyst, nitrogen oxide purifier that is disposed downstream of the three-way catalyst, an upstream oxygen sensor that is disposed between the three-way catalyst and the nitrogen oxide purifier, and a downstream oxygen sensor that is disposed downstream of the nitrogen oxide purifier. An abnormality determination criterion is set in accordance with the exhaust gas amount of the internal-combustion engine wherein the criterion is equivalent to a time period until the output of the downstream oxygen density sensor is read since the output of the upstream oxygen density sensor is changed to rich after the air-fuel ratio of the exhaust gas was over from lean to rich. Any abnormality of the nitrogen oxide purifier is determined when the output of the downstream oxygen density sensor has been changed to rich before the time period represented by the criterion elapses.</p> |
