Industrial Automation Troubleshooting — real-world interview & field troubleshooting questions with clear, practical answers. Tap any question to open its answer. Showing the first 70 questions free. The complete question bank is available as a PDF below. PLC HARDWARE, POWER SUPPLY & CPU DIAGNOSTICS Q1 A PLC CPU enters 'STOP' mode and the 'SF' (System Fault) / Red Fault LED turns solid red. What is the immediate diagnostic procedure? Answer Connect programming software, open the CPU Diagnostic Buffer, and read the exact hexadecimal event code and timestamp. Check for hardware addressing conflicts, missing I/O rack modules, firmware watchdog timeouts, or programming execution errors such as divide-by-zero in an OB block. Q2 A 24VDC PLC switch-mode power supply (SMPS) repeatedly shuts down and 'hiccups' every 2 seconds. How do you locate the fault? Answer Hiccup mode indicates an overcurrent or direct short circuit on the 24VDC distributio...