Abstract: An apparatus for diagnosing and indicating operational failure in automobiles includes a diagnostic circuit for receiving signals input through wiring at both ends of each fuse and wiring of a relay in a fuse box or junction box installed in an automobile and diagnosing operation failure by detecting a change in the logic value of the input signal, and an output device for receiving the result of diagnosis from the diagnostic circuit and outputting signals indicating the location of a defective fuse or relay. Thus, a driver having little knowledge on car maintenance can easily take actions for repair or an emergency measure of automobiles according to the result of diagnosis of operational failure of the automobile informed in an audio-visual manner. Also, automobile repair experts or shops can save time needed for diagnosing and performing accurate repair using the apparatus for diagnosing and indicating operational failure in automobiles according to the present invention.