The present invention deals with diagnosing of a flow system, using status signals carrying information about said flow system, whereby new status information is generated, which relates to a plurality of aspects of the flow system based on said status signals and dependent on a functional model and on a set of predetermined rules. The invention comprises a diagnostic apparatus (20) arranged to analyze the status signals for example by means of an alarm analyzer (23a) arranged to separate alarms into primary and secondary alarms, a sensor fault detector (23b) arranged to detect sensor values that probably are wrong or a fault diagnozer (23c) arranged to find the root causes for unfulfilled goals. Further the invention can comprise a failure mode analyzer (23d) arranged to detect functions that can stop working in the future or a process measurement analyzer (23e) arranged to analyze the status signals of a specific process.