Patent attributes
A deterioration diagnostic system of an exhaust gas purifying catalyst includes: an upstream side switching frequency calculator calculating the number of times of switching on the upstream side; a reference value calculator calculating a downstream side rich/lean determination level having a first width; a downstream side rich/lean determination level adjuster setting the first width to a second larger width when the number of times of switching on the upstream side is smaller than a predetermined value; a downstream side switching frequency calculator calculating the number of times of switching on the downstream side; a frequency ratio calculator calculating a frequency ratio between the number of times of switching on the downstream side and the number of times of switching on the upstream side; and a deterioration determiner determining the deterioration of the exhaust gas purifying catalyst when the frequency ratio is larger than a predetermined value.
