That's a great question! When a machine stops unexpectedly but all the status lights look normal, experienced maintenance engineers use a systematic approach to figure out if it's a hardware problem or a programming bug. Here's how they typically work through it:

First, they check if the PLC is actually in RUN mode - sometimes it's just stuck in PROGRAM or STOP mode, which makes everything look normal but nothing works. Then they connect to the PLC with programming software to see if there are any hidden error codes or diagnostic messages that aren't showing on the status lights.

For hardware issues, they'll test the I/O modules by forcing inputs ON temporarily (safely!) to see if outputs respond. They also check wiring connections, use a multimeter to verify power and signals, and look for loose connections or damaged cables. Hardware problems often show up as inconsistent behavior - sometimes things work, sometimes they don't.

For programming bugs, they'll go online with the PLC program and trace through the logic step by step. They look for things like wrong NO/NC contact selections, overlapping outputs, incorrect I/O address mapping, or scan cycle timing issues. Programming errors usually show consistent patterns - the same thing fails the same way every time.

The key difference is that hardware failures often have physical evidence (burn marks, loose wires, damaged components) while logic bugs are clean but the program doesn't behave as expected. Experienced engineers also check the machine's history - if this happened after a program change, it's likely software; if it happened after maintenance or physical work, it's probably hardware.