Abstract: A document production apparatus has a plurality of replaceable units and a diagnostic system for determining the condition of the replaceable units. A computer control system monitors and controls document production functions, and includes a display for providing instructions for error diagnostic sequences. A diagnostic microprocessor for processing diagnostic signals representative of document production device conditions, has data entries, outputs, and storage for diagnostic routines. Error isolation diagnostic procedures identify suspect replaceable units. The procedures include displaying (1) a list of symptoms from which a user can select the symptom which best describes a condition of the document production apparatus, (2) a stored list of suspect replaceable units which are associated with a selected symptom, and (3) the status of each suspect replaceable unit displayed. The list of suspect replaceable units are displayed in an order of the probability of defect.