An apparatus for checking a trouble in a communication circuit connected between engine and transmission control units. A check signal is produced at uniform intervals of time. The check signal is converted into a predetermined code. The converted code is transmitted through the communication circuit from one of the engine and transmission control units to the other control unit. The code transmitted to the other control unit is monitored to produce a failure signal to indicate that a trouble occurs in the communication circuit when the monitored code is not identical with the predetermined code.