A diagnostic system wherein a central diagnostic center receives sensor data relative to the operating condition of a plurality of remote plants. Each plant records just certain sensor signals and transmits them at respective prescheduled transmisssion times. If certain activation limits are exceeded, a data link with the diagnostic center is immediately established so that the data may be transmitted ahead of its normally scheduled transmission time in order that an immediate diagnostic analysis be made on the plant. A communication link between the plant and diagnostic center is kept open for a predetermined period of time in which more data may be sent from the plant to the diagnostic center, and results and instructions, sent from the diagnostic center back to the plant. Such instructions may include the changing of certain parameters in the examination of the sensor signals.