ControlLogix Major Fault Type 3 Code 23 at Run: Causes and Fix

Q: What causes Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T03:C23?

1. Controller switched to Run before I/O finished connecting: Watch the controller I/O indicator; switch to Run only once it is solid green. 2. Remote I/O powers up slower than the controller: Compare power-up times of the remote racks and network devices with the controller limit. 3. A required module that is missing or cannot connect at all: Treat as T03:C16: check the module, keying and network path in the Connection tab.

At least one I/O module configured for "Major Fault On Controller If Connection Fails While in Run Mode" had not established its connection before the controller went to Run, within the controller’s time limit. Rockwell lists 30 s (5380/5580, non-redundant), 90 s (5380/5580 redundant), 60 s (5590 non-redundant) and 120 s (5590 redundant). Rockwell’s action: wait for the controller I/O indicator to turn green before changing to Run mode.

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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

On the change from Program to Run
At power-up in Run mode, when I/O takes longer to connect than the controller limit

Likely causes and checks

1Controller switched to Run before I/O finished connecting

Points to it: Fault right after a download or mode change; I/O indicator not yet green.
Check: Watch the controller I/O indicator; switch to Run only once it is solid green.

2Remote I/O powers up slower than the controller

Points to it: Fault after plant power-up; remote racks or devices come online after the time limit.
Check: Compare power-up times of the remote racks and network devices with the controller limit.

3A required module that is missing or cannot connect at all

Points to it: The same module never connects; the fault repeats on every attempt.
Check: Treat as T03:C16: check the module, keying and network path in the Connection tab.

Telling it apart

T03:C23 is raised at the transition to Run; a required connection lost after the controller is already running gives T03:C16.
If the module connects a few seconds later without any change, the cause is timing; if it never connects, it is the module or its path.

Relevant parameters

Parameter	Name	Why it matters
Module Properties > Connection	Major Fault On Controller If Connection Fails While in Run Mode	Only modules with this set can raise T03:C23.

Evidence to collect

Major Faults tab screenshot
Controller I/O indicator state at the change to Run
Connection tab of the modules that were not connected
Power-up sequence of remote racks and network equipment

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Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T03:C23: I/O fault: required connection not established before RUN