The invention relates to the integration of diagnostics and control in a production line based on process components (10). Preferably, the control system and the diagnostic system are built as a hierarchical and highly distributed system, where as much as possible of the pre-processing and low-level diagnostics is accomplished at the lower levels of the system. At the process component level, diagnostics (12) are integrated with the control software (11) controlling the process hardware of the production line. The production line is typically comprised of a number of cells of process components, and each cell is controlled by a cell controller (20). At the cell level, diagnostics (22) are integrated in the cell control software (21) of the cell controllers (20). For example, diagnostic functions could be integrated with the control software programs by extending the IEC 1131-3 SFC language with diagnostic functions, or pre-compiling the source code of the control program and adding code for diagnostic functions.