ControlLogix and CompactLogix I/O and Connection Fault Codes: 16#0204, 16#0203 and Other Common Codes

When a ControlLogix or CompactLogix controller loses an I/O module, Logix Designer shows a fault code such as 16#0204 or 16#0203. The code tells where to look. This article lists what the common I/O and connection codes mean and what to check, using Rockwell Automation's own descriptions.

Where the fault shows

• The controller's I/O status indicator flashes, and a yellow warning symbol appears on the module in the I/O Configuration tree of Logix Designer (1756-PM014O, chapter 3, "Indications of I/O faults").
• The fault code and its description are shown on the Connection tab of the module's properties (same chapter).
• On a ControlLogix 5580 controller the status display shows messages in the form "I/O Fault Local:3 #0107 Connection Not Found"; with several I/O faults the controller indicates the first fault reported (1756-UM543, "Fault Messages").
• A lost module stops the controller (major fault) only if the module is configured to raise a major fault when its connection fails; otherwise the controller keeps running with the module faulted (1756-UM543). In that case the "Major Faults" tab shows Type 03 Code 16 (the connection to a required module failed) — see Logix major fault codes: reading Type and Code.

Reading the code: 0x0204, 16#0204 and #0204 are the same

The same code can be written three ways:

• The 1756-RD001 spreadsheet ("IO" tab, column "Fault Code (I/O Map Object)") writes it as 0x0204.
• Logix Designer shows the full 16#0204 on the Connection tab.
• Some places, such as the 5580 status display, show only the last characters, #0204 (1756-PM014O, chapter 3).

All three are the same hexadecimal number. When searching 1756-RD001, type the 0x0204 form and set the search to the whole workbook, not just the open tab.

16#0204 — Connection request timed out

Rockwell describes this code as the target module not responding, and also appearing to be missing from the backplane or network (Logix Designer help, "Module Faults: 16#0200 – 16#02ff"). The checks Rockwell lists:

1. The module has not been removed, is still working and is receiving power.
2. The correct slot number is specified in the I/O configuration.
3. The module is properly connected to the network.
4. For FLEX I/O, the correct terminal base is in use.

16#0203 — Connection timed out

Here the controller does see the module on the network or backplane, but the module stops responding as expected; when this fault occurs the controller usually tries to remove and remake the connection continuously (same help page). Rockwell's example is Ethernet multicast packets that do not come back, so check the network path between the controller and the module: cables, connectors and switch ports.

Common I/O and connection codes

Explained from the "IO" tab of 1756-RD001 (September 2025) and, for the keying codes, from the Logix Designer help pages cited below; the last column is what to check first.

Checks in order

1. Write down the code and any extra information on the Connection tab before changing anything. For 16#0009 an additional hex code points to the wrong configuration parameter; for 16#0109 and 16#0117 the extra information is the tag name of the failed connection (1756-RD001).
2. One module or a group? If several modules behind the same communication module fault together, check that module and its network path first (16#0800, 16#0203, 16#0302) before suspecting each I/O module.
3. Fault right after a module swap or a new download points to keying and configuration mismatch (16#0114, 16#0116, 16#0107, 16#0205).
4. Controller-to-controller (produce/consume) connections: check tag names, data types, number of consumers and RPI (16#031F, 16#0117, 16#0801).
5. Fault that comes and goes: look at the network first — cables, connectors, switch ports, load.

Clearing the fault safely

For connection faults the controller keeps trying to reconnect; Rockwell states this explicitly for 16#0203. So as soon as the module is refitted, the cable reconnected or the configuration corrected, the module can receive output data again and the machine can start moving at once. Before working on it, isolate the affected equipment under the site's lockout and permit-to-work rules.

If the module is configured to raise a major fault, the controller stops on Type 03 Code 16; clearing the major fault and switching to Run restarts the program. For Type 03 Code 23 (a required connection not established before going to Run), 1756-RD001 advises waiting for the controller's I/O indicator to turn green before switching to Run.

When it is hardware

Most codes in the table are configuration or connection problems, not a failed module. A few codes in 1756-RD001 point directly at hardware:

• Type 03 Code 20 or 21: ControlBus chassis information cannot be read and the chassis may be bad — not recoverable, replace the chassis.
• 16#0336: the module cannot read its removable terminal block (RTB) information — replace the RTB.
• 16#FE12: automatic firmware update failed — check the status indicators and troubleshoot the module.

If 16#0204 persists while the module has power, the right slot and the right network, try a spare of the same catalog number and revision (if available): if the fault follows the module, the module needs testing or repair through Rockwell's channels.

The full list

Rockwell publishes every Logix 5000 fault code in the Logix 5000 Controller and I/O Fault Codes spreadsheet, 1756-RD001 (referenced in 1756-PM014O and 1756-UM900). Its "IO" tab (September 2025) holds almost 150 codes with corrective actions.

Before calling for help

Note the exact fault code and text from the Connection tab (including any extra information), the module catalog number and slot, the controller and Logix Designer versions, whether the fault is permanent or comes and goes, and whether one module or a group is affected. A copy of the project file and a photo of the rack and network switch help TDL engineers find the cause faster.

Need an engineer on site: request support.