A fault diagnosing method for diagnosing an engine by tracking down a faulty region in the event of a misfire that occurs in a multicylinder engine having a misfire detecting function, using an external diagnosing machine that communicates with an engine control unit; characterized in that the method comprises the steps of: reading a diagnostic fault code representative of a misfiring cylinder from the engine control unit into the external diagnosing machine; detecting misfire patterns and classifying the misfire patterns depending on whether or not successive misfires occur in the misfiring cylinder; and tracking down faulty regions according to the detected misfire patterns; wherein the step of detecting the misfire patterns and classifying the misfire patterns further comprises the steps of: idling the engine in accordance with an instruction from the external diagnosing machine in an attempt to repeat a misfire in the misfiring cylinder; and if a misfire is repeated in the misfiring cylinder, detecting the repeated misfire pattern and classifying the repeated misfire pattern; or if a misfire is not repeated in the misfiring cylinder, detecting the misfire pattern based on successive data generated upon occurrence of a misfire in the misfiring cylinder, which were stored in the engine control unit at a time that the diagnostic fault code was generated, said data comprising the number of misfires which occurred within a predetermined period, and classifying the misfire pattern. reading a diagnostic fault code representative of a misfiring cylinder from the engine control unit into the external diagnosing machine; detecting misfire patterns and classifying the misfire patterns depending on whether or not successive misfires occur in the misfiring cylinder; and tracking down faulty regions according to the detected misfire patterns; wherein the step of detecting the misfire patterns and classifying the misfire patterns further comprises the steps of: idling the engine in accordance with an instruction from the external diagnosing machine in an attempt to repeat a misfire in the misfiring cylinder; and if a misfire is repeated in the misfiring cylinder, detecting the repeated misfire pattern and classifying the repeated misfire pattern; or if a misfire is not repeated in the misfiring cylinder, detecting the misfire pattern based on successive data generated upon occurrence of a misfire in the misfiring cylinder, which were stored in the engine control unit at a time that the diagnostic fault code was generated, said data comprising the number of misfires which occurred within a predetermined period, and classifying the misfire pattern. idling the engine in accordance with an instruction from the external diagnosing machine in an attempt to repeat a misfire in the misfiring cylinder; and if a misfire is repeated in the misfiring cylinder, detecting the repeated misfire pattern and classifying the repeated misfire pattern; or if a misfire is not repeated in the misfiring cylinder, detecting the misfire pattern based on successive data generated upon occurrence of a misfire in the misfiring cylinder, which were stored in the engine control unit at a time that the diagnostic fault code was generated, said data comprising the number of misfires which occurred within a predetermined period, and classifying the misfire pattern.