System for diagnosing engine exhaust gas purifying device and system for diagnosing sensor

A system for diagnosing a sensor in an apparatus having a response characteristic of the output signal which changes with the change in the condition of the sensor per se, comprising;

• feature wave form extracting means for attenuating signals in a frequency band lower than a control frequency band among the output signals from said sensor;
• autocorrelation function calculating means for calculating and outputting the autocorrelation function .phi.xx of the output signal from said sensor, said autocorrelation function calculating means being adapted to calculate the autocorrelation function .phi.xx from the signal which has passed said feature wave form extracting means, by using data obtained at predetermined intervals among the data obtained by said sensor;
• sensor condition determining means having a preset reference value for determining the condition of said sensor by comparing the value of said autocorrelation function .phi.xx with said preset reference value;
• and
• means for feedback controlling the apparatus having a condition detected by said sensor in accordance with the detection result of said sensor.

A system for diagnosing an engine exhaust gas purifying device for purifying with a catalyst an exhaust gas from an engine having an air/fuel ratio control unit for adjusting the fuel injection quantity by detecting the concentration of the oxygen or the air/fuel ratio in the exhaust gas so that a constant air/fuel ratio in the exhaust gas is maintained, comprising:

• a front air/fuel ratio sensor for detecting the oxygen concentration or the air/fuel ratio in the exhaust gas prior to the purification thereof with said catalyst;
• a rear air/fuel ratio sensor for detecting the oxygen concentration or the air/fuel ratio in the exhaust gas after the purification thereof with said catalyst;
• a processor coupled to receive output signals from the front and rear air/fuel ratio sensors, said processor determining the deterioration condition of the catalyst and diagnosing the engine exhaust gas purifying device based upon said deterioration condition of the catalyst, the processor including:
• at least one feature wave form extraction that attenuates signals in a frequency band lower than the air/fuel ratio control frequency band of the air/fuel ratio control unit in signal outputs from the front and rear air/fuel ratio sensors;
• and a correlation function calculator that calculates a correlation function of the signal which has passed through the feature wave form extractor.