ControlLogix Major Fault Type 3 Code 16 (I/O Connection): Causes

Q: What causes Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T03:C16?

1. Module removed, unpowered or failed: Check the module is seated in the chassis and powered; read its status indicators and the Module Properties Connection tab. 2. Electronic keying mismatch after module replacement: Compare the installed module catalog number and firmware with the I/O configuration and its keying setting. 3. Network path to remote I/O lost (EtherNet/IP, ControlNet, adapter or switch): Check the adapter, cabling, switch ports and the communication module path in the I/O tree.

A connection to an I/O module failed, and that module is configured so that a connection failure in Run mode causes a major fault on the controller. Rockwell’s checks are that the module is in the chassis, that the electronic keying requirements are met, and the Major Faults tab and the module’s Connection tab for details.

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When it happens

In Run mode, when a required module or its network path drops
After a module is removed, replaced or loses power

Likely causes and checks

1Module removed, unpowered or failed

Points to it: The module shows a fault status or is dark; only one module is named.
Check: Check the module is seated in the chassis and powered; read its status indicators and the Module Properties Connection tab.

2Electronic keying mismatch after module replacement

Points to it: Fault started after a module swap; Connection tab reports a keying or identity mismatch.
Check: Compare the installed module catalog number and firmware with the I/O configuration and its keying setting.

3Network path to remote I/O lost (EtherNet/IP, ControlNet, adapter or switch)

Points to it: Several modules in the same remote chassis or rack lose connection together.
Check: Check the adapter, cabling, switch ports and the communication module path in the I/O tree.

Telling it apart

T03:C16 happens while running; T03:C23 happens at the change to Run when a required connection was never established.
Only modules with "Major Fault On Controller If Connection Fails While in Run Mode" set raise this major fault; other modules lose connection with an I/O fault indication but the controller keeps running.
One module named points to that module or slot; a whole remote rack points to the network path or adapter.

Relevant parameters

Parameter	Name	Why it matters
Module Properties > Connection	Major Fault On Controller If Connection Fails While in Run Mode	When set, a lost connection to this module faults the controller (Type 3).
Module Properties > Electronic Keying	Electronic keying	Rockwell lists keying requirements as a check; a mismatched replacement module is refused.

Evidence to collect

Major Faults tab screenshot (names the module)
Module Properties Connection tab and module status indicators
I/O configuration tree from the program export
Catalog number and firmware of the installed module
Network switch or adapter diagnostics for remote I/O

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Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T03:C16: I/O fault: connection failed on a required module