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PLC fault code troubleshooting

PLC controller faults by maker and model: Allen-Bradley ControlLogix and CompactLogix major faults, Siemens S7-1500 error OBs, Beckhoff TwinCAT ADS errors and EtherCAT AL status codes. Each guide gives the meaning, the ranked causes and the check that confirms each one, and cites the manufacturer manual it is based on.

By controller

Rockwell Automation / Allen-BradleyControlLogix / CompactLogix (Logix 5000)

12 fault-code guides

SiemensSIMATIC S7-1500

6 fault-code guides

BeckhoffTwinCAT 3 (XAR/XAE) with EtherCAT

20 fault-code guides

Manufacturer documentation

The guides on this page are written from 50 manufacturer manuals and 8 manufacturer web pages (alarm lookups, product pages), and each guide cites the section it is based on. 48 of these manuals are indexed in the InduQT manual library, so AI diagnoses search them and cite the page. Links go to the manufacturer's copy.

Overview

A PLC stops a machine in one of two ways: the controller faults and drops out of run, or a module, network connection or program step fails and the logic stops the sequence. Controllers keep a fault log or diagnostic buffer that says which, and where in the program or I/O tree it happened.

Common problems

Controller faulted (major fault, CPU in STOP)

Read the fault type and code, or the diagnostic buffer entry, before resetting. It names the task, program and routine, or the module, that caused it; clearing the fault without it throws that evidence away.

I/O module or network connection lost

Check whether one module, one rack or the whole network dropped, and whether it coincided with a power dip, a cable being moved or a device being replaced.

Watchdog or cycle time exceeded

A task ran longer than its limit. Look for a loop, a large array operation or a recent program change, and compare scan times before and after.

Program error after a change

Array index out of range, unknown instruction or parameter mismatches usually follow a download. Compare with the last good program version.

Evidence that helps

  • Photo of the controller status or fault screen
  • Fault log or diagnostic buffer export
  • Controller model and firmware version
  • What changed recently: downloads, modules, network devices
  • Screenshot of the routine named in the fault
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