Patent ID: 6836722
Filing Date: 2004-12-28
Classification: F02D

Abstract:
A system for diagnosing a failure of a rear oxygen sensor of a vehicle comprising:a control unit; a vehicle speed sensor communicating with the control unit and generating a signal responsive to a vehicle speed; an engine speed sensor communicating with the control unit and generating a signal responsive to an engine speed; an air flow sensor communicating with the control unit and generating a signal responsive to an air flow rate; a throttle position sensor generating signals responsive to a throttle position; a front oxygen sensor and a rear oxygen sensor generating signals responsive to oxygen content in exhaust emission, the front and rear oxygen sensor being disposed upstream and downstream, respectively, of a catalytic converter and communicating with the control unit; and a fuel injector for injecting fuel according to injection command signals from the control unit based on said signals generated by said sensors; wherein the control unit is programmed to execute a diagnostic program comprising determining whether a predetermined monitoring condition exists: determining whether a vehicle speed is 0; determining whether a difference between a maximum value and a minimum value of monitored signals of the rear oxygen sensor is less than a first value; determining whether the difference is between the first value and a second value, the second value being greater than the first value, if the difference is not less than the first value; injecting fuel for a predetermined period such that an air/fuel ratio becomes a predetermined air/fuel ratio, the predetermined air/fuel ratio being richer than a stoichiometric air/fuel ratio; if the difference is between the first value and the second value, detecting an output value of the front oxygen sensor and an output value of the rear oxygen sensor after injecting fuel for the predetermined period; and determining whether the detected output value of the front oxygen sensor is greater than a first threshold value, wherein a front air/fuel mixture is determined to be rich if an output value of the front oxygen sensor is greater than the first threshold value, and the detected output value of the rear oxygen sensor is less than a second threshold value, and wherein a rear air/fuel mixture is determined to be rich if an output value of the rear oxygen sensor is greater than the second threshold value.