What is your methodology for troubleshooting PLC-based systems?

Systematic troubleshooting approach: (1) Gather information - operator reports, alarm history, recent changes, (2) Observe - check HMI indicators, PLC status LEDs, controller faults, (3) Verify power and communication - module status, network connectivity, (4) Check I/O - force or monitor individual points, verify field wiring with multimeter, (5) Analyze program - online monitoring, check logic conditions, review scan times, (6) Test hypothesis - make changes in controlled manner, (7) Document findings and corrective actions. Use diagnostic tools: PLC software online monitoring, cross-reference reports, trend data, and event logs.