The present invention relates to a system for checking the function of a catalyst used to purify the exhaust gas in an internal combustion engine. In particular, the invention is concerned with a function diagnostic system for an exhaust gas purifying apparatus in an internal combustion engine, which diagnostic system has suitable for evaluating the exhaust gas purifying performance for a specific component.
Heretofore there has been adopted a method in which O.sub.2 sensors are disposed upstream and downstream of a catalyst, and whether the catalyst, is deteriorated or not is judged by comparing the outputs of both sensors. In this case, it is actually the O.sub.2 storage capacity out of the various catalyst functions that is measured, rather than the conversion efficiency of each exhaust gas component.
According to the above conventional method, when the conversion efficiency is determined independently of the O.sub.2 storage capacity, it is impossible to make an accurate detection of the conversion efficiency. For solving this problem, the use of a sensor for detecting a specific exhaust gas component, instead of an O.sub.2 sensor, may be effective in measuring the conversion efficiency of that exhaust gas component. An example of using an HC sensor for detecting the conversion efficiency of HC (hydrocarbon) is disclosed in Japanese Patent Laid Open No. 109021/92. According to the method disclosed therein, however, it is necessary to dispose HC sensors both upstream and downstream of a catalyst. Therefore, the environmental conditions, particularly around the upstream-side HC sensor, are severe (for example, high temperature and high exhaust gas component concentrations), thus resulting in the sensitivity and output characteristic of the sensor itself being deteriorated. This eventually exerts a bad influence on the diagnostic accuracy for catalyst deterioration.