A diagnostic monitoring system, for detecting and displaying malfunctions or abnormal operations occurring in various engine operation control computers, includes a microcomputer, actuators and sensors. Each of the engine operation control computers performs a diagnosis program as a part of their control programs and produces a signal whenever a malfunction or abnormal operation is detected. A central supervisory system receives such signals from the engine operation control computers and stores them separately for the individual control computers. A display unit displays the stored signals in response to instructions from the central supervisory system.