Abstract: An apparatus for accurately and promptly determining a failure of an oxygen concentration sensor. The oxygen concentration sensor is arranged at a location downstream of a catalyst in an exhaust system of an internal combustion engine. The apparatus measures a time period elapsed after termination of a fuel cut-off operation, and calculates an integrated amount SUMSVS of exhaust gases exhausted after the termination of the fuel cut-off operation. A failure of the oxygen concentration sensor is determined depending on whether or not a signal value SVO2 of a signal from the oxygen concentration sensor has crossed a predetermined threshold value #SVO2CHK. Further, when a predetermined time period #TMFCCKD has elapsed, if the signal value SVO2 has not crossed the predetermined threshold value #SVO2CHK, and at the same time when the integrated amount SUMSVS of the exhaust gases has not reached a predetermined value #SUMSVAFC the failure determination is suspended.

Abstract: There is provided an apparatus for determining a failure of an oxygen concentration sensor, which is capable of determining a failure of the oxygen concentration sensor accurately and promptly. The oxygen concentration sensor 25 is arranged at a location downstream of a catalyst 8 in an exhaust system of an internal combustion engine 3. The apparatus 1 measures a time period elapsed after termination of a fuel cut-off operation, and calculates an integrated amount SUMSVS of exhaust gases exhausted after the termination of the fuel cut-off operation (step S13). A failure of the oxygen concentration sensor is determined depending on whether or not a signal value SVO2 of a signal from the oxygen concentration sensor 25 has crossed a predetermined threshold value #SVO2CHK (steps S51, 55).

Abstract: An improved method of determining activation of an exhaust gas concentration sensor for detecting the concentration of an exhaust gas ingredient in exhaust gases from an internal combustion engine. The sensor comprises an exhaust gas ingredient-detecting element, and a heater for heating the element. An output signal from the sensor is used for air-fuel ratio feedback control of an air-fuel mixture supplied to the engine. It is determined that activation of the sensor has been completed when an output voltage from the sensor has become lower than a predetermined activation-determining reference voltage. The predetermined activation-determining reference voltage is changed depending on a time period over which the heater has been energized while the engine is in an operating condition under which the air-fuel ratio feedback control should be carried out.