Catalyst degradation diagnosing apparatus for air fuel ratio control

Abstract

To reliably diagnose catalyst degradation in an air fuel ratio control system without being subjected to the influence of engine operating conditions, a front O.sub.2 sensor output VFO and a rear O.sub.2 sensor output VRO are detected on both the upstream and downstream sides of catalytic converter; air fuel ratio feedback correction coefficients .alpha. are determined on the basis of basic feedback control constants P.sub.R,L or i.sub.R,L and correction values PHOS according to rich and lean air fuel ratio conditions; periods T and amplitudes .alpha..sub.R -.alpha..sub.L of the air fuel ratio feedback correction coefficient .alpha. are measured; a rich discriminating catalyst diagnosing slice level RSLH2 is set higher than a rich discriminating air fuel ratio feedback controlling slice level RSH1 and a lean discriminating catalyst diagnosing slice level RSLL2 is set lower than a lean discriminating air fuel ratio feedback controlling slice level RSLL1 in such a way that RSLH2 increases and RSLL2 decreases with increasing product of the period and amplitude of .alpha.; the number j.sub.F of times when the front sensor output VFO changes across a stoichiometric mixture ratio SL.sub.F is connected; the number j.sub.R of times when the rear sensor output VRO changes across the slice levels RSLH2 and RSLL2 is counted; and the catalyst is determined to be degraded if j.sub.R /j.sub.F .gtoreq.k, where k<1.