• A reasonable troubleshooting guide (note: not debugging),
Look at the process and see if it is in a normal state. i.e. no jammed actuators, broken parts, etc. If there are visible problems, fix them and restart the process.
Look at the PLC to see which error lights are on. Each PLC vendor will provide documents that indicate which problems correspond to the error lights. Common error lights are given below. If any off the warning lights are on, look for electrical supply problems to the PLC.
HALT: something has stopped the CPU
RUN: the PLC thinks it is OK (and probably is)
ERROR: a physical problem has occurred with the PLC
Check indicator lights on I/O cards, see if they match the system. i.e., look at sensors that are on/off, and actuators on/off, check to see that the lights on the PLC I/O cards agree. If any of the light disagree with the physical reality, then interface electronics/mechanics need inspection
Consult the manuals, or use software if available. If no obvious problems exist the problem is not simple, and requires a technically skilled approach.
If all else fails call the vendor (or the contractor) for help.