Patent ID: 7549284

Claim:
A diagnostic device of an engine exhaust purifying system for diagnosing an air-fuel ratio sensor of an internal combustion engine comprising: a plurality of cylinder groups; a plurality of first exhaust passages being connected at a downsteam-side of said plurality of cylinder groups respectively; a plurality of pre-catalyst being disposed in each of said plurality of first exhaust passages and being made of a three-way catalyst; a plurality of downsteam air-fuel ratio sensor being disposed at downsteam-side of said plurality of pre-catalyst respectively; a second exhaust passage being merged together with said plurality of first exhaust passages; and a main catalyst being disposed at said second exhaust passage and being made of a three-way catalyst; wherein said diagnostic device diagnoses said downsteam air-fuel ratio sensor by detecting a response of said downsteam air-fuel ratio sensor, when switching an air-fuel ratio of exhaust gas flowing into said plurality of cylinder groups as rich/lean and while keeping an air-fuel ratio of exhaust gas flowing into said main catalyst near a stoichiometric ratio, wherein said diagnostic device performs the air-fuel ratio control for each said cylinder group such that the output of at least one of said downstream air-fuel ratio sensors becomes rich, and wherein said diagnostic device controls the air-fuel ratios in all said cylinder groups to be temporarily rich at the start of diagnosis.