ControlLogix Major Fault Type 4 Code 16 (Unknown Instruction)

Q: What causes Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T04:C16?

1. Program converted from another controller or platform: Review the conversion report and search the routines for instructions flagged during conversion. 2. Project and controller firmware do not match the instruction set used: Compare the project revision, controller firmware and Logix Designer version.

The controller encountered an unknown instruction while executing the program. Rockwell’s corrective action is to remove the unknown instruction and notes that this probably happened during a program conversion process.

Diagnose this with AI

Upload a keypad photo, the fault history or a parameter export from your Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000). InduQT compares the evidence and suggests the next useful checks.

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When it happens

When the routine containing the instruction first executes after a download
After a project was converted from another platform or firmware revision

Likely causes and checks

1Program converted from another controller or platform

Points to it: Project was recently migrated (e.g. from PLC-5/SLC or another Logix revision).
Check: Review the conversion report and search the routines for instructions flagged during conversion.

2Project and controller firmware do not match the instruction set used

Points to it: Fault started after a firmware change or download from a different Logix Designer version.
Check: Compare the project revision, controller firmware and Logix Designer version.

Telling it apart

Like all Type 4 faults it comes from instruction execution, so the program’s fault routine sees it before the Controller Fault Handler.
The fault-specific information in MajorFaultRecord and the Major Faults tab identify the program and routine; start there instead of searching the whole project.

Evidence to collect

Major Faults tab screenshot
Program export (L5X) of the routine named in the fault
Conversion or migration report
Controller firmware and Logix Designer version

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Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T04:C16: Program fault: unknown instruction encountered