Patent ID: 7055310

Claim:
A purifier deterioration determining apparatus for an internal-combustion engine having an exhaust gas purifier disposed in an exhaust pipe of the engine, and a downstream exhaust gas sensor disposed in the exhaust pipe on the downstream side of the purifier for generating outputs corresponding to components of an exhaust gas from the engine, said apparatus comprising; means for controlling an air/fuel ratio of the engine to a predetermined value based on the outputs of the downstream exhaust gas sensor; extracting means for performing a non-linear filtering to extract rectangle-shaped waveform components, which correspond to the normal outputs of the exhaust gas sensor, from the outputs of the downstream exhaust gas sensor during an operation under an air/fuel ratio control; means for subtracting the rectangle-shaped waveform components from the sensor outputs to extract noise components representing a degree of the catalyst deterioration; and means for determining deterioration of the purifier.