Abstract: A diagnostic system for use in troubleshooting malfunctions in an aircraft engine control system receives sensor data obtained during a flight. The sensor data is supplemented by signals indicative of the operation of the engine control system's computer. The combined data is downloaded to the diagnostic system. Included in the diagnostic system is an inference engine and knowledge database. The diagnostic system includes a computer for executing a plurality of algorithms by means of the inference engine using the downloaded data and knowledge database as input to aid the technician in correctly determining the source of malfunctions in a minimum of time. The diagnostic system is capable of processing "unknown" responses to diagnostic system inquiries and can retrocede a step or steps during the execution of diagnostic system algorithms.