By Dovydas Kersys · Chapter 52 of 58 in Machine Commissioning.
Before you begin: Follow the machine documentation, risk assessment, site procedures and lockout/tagout requirements. Commissioning can expose personnel to electrical energy, stored pressure and unexpected motion. Never defeat a safety function merely to make a test easier. Examples and diagrams are generic and must be checked against the actual machine.
Symptom
A photoelectric sensor LED changes when product is present, but the PLC input tag never changes.
Diagnostic approach
The fault must be split into layers: sensor power/output, field wiring, terminal, input module and tag mapping.
What the team found
A meter shows the sensor output reaches the terminal block, but not the PLC input terminal. The drawing reveals a last-minute terminal reroute that was marked on paper but not wired to the module. The wire is corrected, the input loop is retested, and the redline is added to the as-built set.
Commissioning lesson
A sensor LED proves only the sensor. Loop checking must follow the entire path.
Use the same layered isolation method even when the symptom seems to point at software.
FIELD RULE: Preserve evidence before making changes: values, states, diagnostics, timestamps and the exact machine condition.
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Adapted from Machine Commissioning for Maintenance and Automation Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.