Troubleshooting
PLC Troubleshooting
PLC Troubleshooting in industrial control systems — covering 6 topic areas in the CurryControls.com controls and automation knowledge hub.
Written guides in this section
Full references, procedures, and diagnostic guides published under PLC Troubleshooting.
Processor Faulted
A PLC with its fault light on and no logic running. How to read what it is telling you, the causes in order of likelihood, and how to restart it without repeating the fault.
8 min readUpdated Sep 5, 2026
Outputs Not Energizing
The program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
8 min readUpdated Sep 5, 2026
Inputs Not Reading
A switch closes in the field and the program never sees it. How the wetting voltage, the common, sinking and sourcing, and the wiring each lose the signal, and how to find which one did.
8 min readUpdated Sep 5, 2026
Logic Not Executing As Expected
The rung looks right and the plant does something else. The scan-order, duplicate-output, unscheduled-routine, and data-type traps behind it, and how to find which one you are in.
8 min readUpdated Sep 5, 2026
Program Will Not Download
The software refuses to download, or the download starts and fails. The version, path, mode, memory, and protection problems behind it, and the order to check them in.
7 min readUpdated Sep 5, 2026
Retentive Data Lost
The plant came back from a power cycle with zero setpoints, zero totals, and the wrong pump as lead. Which backup failed, which restore overwrote what, and how to get the data back and keep it.
7 min readUpdated Sep 5, 2026
Topics in this area
Processor Faulted
Processor Faulted — practical reference notes for controls engineers, integrators, and technicians working with plc troubleshooting.
Outputs Not Energizing
Outputs Not Energizing — practical reference notes for controls engineers, integrators, and technicians working with plc troubleshooting.
Inputs Not Reading
Inputs Not Reading — practical reference notes for controls engineers, integrators, and technicians working with plc troubleshooting.
Logic Not Executing As Expected
Logic Not Executing As Expected — practical reference notes for controls engineers, integrators, and technicians working with plc troubleshooting.
Program Will Not Download
Program Will Not Download — practical reference notes for controls engineers, integrators, and technicians working with plc troubleshooting.
Retentive Data Lost
Retentive Data Lost — practical reference notes for controls engineers, integrators, and technicians working with plc troubleshooting.
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