trouble in the sequence circuit such as mismatching of operation timing with reference to fig1 b and 14 a fourth modification of the sequence control system and the method according to the present invention will be described hereinbelow in this fourth modification the system is further provided with a self diagnosis function for analyzing abnormal spots with reference to past abnormal data stored in a data base in comparison the operation of the prior art sequence control system which does not includes the second memory 7 will be explained with reference to a flowchart shown in fig1 b that is first the ladder sequence circuit regarded as being related to abnormal signals under consideration of each section status at manufacturing line stop is displayed on a crt display of the programming device 8 in step 510 next the operator confirms the on off status of the input system such as internal coils relay contacts limit switches etc at each step by observing the crt display in step 511 when no abnormal spot can be located in step 511 the operator returns to the step 510 to repeat the same step in step 512 as described above since the operator must find an abnormal spot on the basis of step status at the manufacturing line and with reference to the sequence circuit diagram there exists a problem in that this requires much skill and.