| 摘要 |
<p>PURPOSE:To provide a diagnostic device with which a deterioration condition of an engine exhaust emission control device can be diagnosed. CONSTITUTION:A front air-fuel ratio sensor 3 and a rear air-fuel ratio sensor 4 for detecting air-fuel ratios upstream and downstream of catalyst 2, and an auto-correlation function calculating means 13 for calculating an auto- correlation function phixx of an output signal of the front air-fuel ratio sensor 3 and outputting the maximum value (phixx)max of the auto-correlation function phixx in each specified period for the period are provided. A cross-correlation function phixy calculating means 14 for calculating a cross-correlation function for the output signal of the front air-fuel sensor 3 and an output signal of the back air-fuel ratio sensor 4, and outputting the maximum value (phixy)max in each specified period for the period, and a means 16 to calculate a ratio (sequential deterioration index PHIi) between (phixy)max and (phixx)max for each specified period, and determining deterioration condition of the catalyst 2 by comparing this value with a preliminarily set reference value are provided.</p> |
