1. Diagnostic system made from observed information ( 5 ) from a process ( 1 ) monitored by a programmable logic controller ( 10 ) provided with at least one processing unit ( 11 ) to run an application program stored in storage means ( 12 ) in the programmable logic controller ( 10 ), characterised in that the diagnostic system comprises: an observation module ( 30 ), stored in said storage means ( 12 ) and run by the processing unit ( 11 ), collecting the observed information ( 5 ) as input, to apply an observation processing ( 31 ) to the observed information ( 5 ), in order to output a processed information ( 35 ) representative of the observed information ( 5 ), a learning module ( 40 ) generating a diagnostic function ( 45 ) from several items of processed information ( 35 ) received from the observation module ( 30 ) during a determined time period, a detection module ( 20 ), stored in said storage means ( 12 ) and run by the processing unit ( 11 ), collecting the observed information ( 5 ) as input, to apply to the observed information ( 5 ) a detection processing ( 21 ) comprising the diagnostic function ( 45 ) derived from the learning module ( 40 ), in order to output a diagnostic result ( 25 ).