Patent ID: 7424801

Claim:
A catalyst-deterioration diagnostic device for diagnosing a deterioration state of a catalyst based on a temperature of the catalyst which purifies engine exhaust gases, comprising an air amount sensor, and means for obtaining from the air amount sensor an amount of air which flows into the engine, calculating an average of the air amounts obtained for a predetermined period of time, and then estimating the temperature of the catalyst from an accumulated quantity of heat which is determined based on the average of the obtained air amounts, further comprising: means for correcting, based on the temperature estimated by the catalyst temperature estimation means, a diagnostic index or a diagnostic reference value used to decide a purifying efficiency of the catalyst; and means for deciding the purifying efficiency of the catalyst based on the corrected, diagnostic index or the corrected diagnostic reference value, and pre-arranged as a catalyst-deterioration diagnostic area are a first area to prohibit a diagnosis a second area to correct the diagnostic index or the diagnostic reference value, and a third area not to correct the diagnostic index or the diagnostic reference value further, are pre-arranged as a catalyst deterioration diagnostic area wherein the correction means correcting the diagnostic index or the diagnostic reference value when the estimated temperature falls in the second area, and the deciding means does not decide the purifying efficiency of the catalyst when the estimated temperature falls in the first area, the deciding means does decide the purifying efficiency of the catalyst using the diagnostic index or the diagnostic reference value corrected by the correction means when the estimated temperature falls in the second area, and the decision means does decide the purifying efficiency of the catalyst using the diagnostic index or the diagnostic reference value not corrected by the correction means when the estimated temperature falls in the third area.