Allen-Bradley Flex I/O Module Fault (1794 Series)

Flex I/O module (1794 series) has faulted. Module may show red status LED. Typically found in remote I/O racks connected via DeviceNet, EtherNet/IP, or ControlNet.

Probable Causes (ranked)

• Module not fully seated on the Flex I/O backplane terminal base
• Module internal hardware failure
• Field wiring fault — shorted or open wiring on an I/O channel
• 1794-AENT or 1794-ADN adapter lost communication with PLC
• 24VDC power supply to the Flex I/O bank is low or failed
• Module configuration mismatch between physical module and PLC I/O tree

Fix Steps

1. 1 Check module status LED — green=OK, red=fault, off=no power or dead
2. 2 Reseat the module on its terminal base — push firmly until it locks in place
3. 3 Check 24VDC power to the Flex I/O bank power supply — measure at terminals
4. 4 Check adapter module (1794-AENT for EtherNet/IP or 1794-ADN for DeviceNet) status LEDs
5. 5 If field wiring suspected, disconnect wiring from the faulted module — if fault clears, trace the field wiring
6. 6 Verify module catalog number matches the PLC I/O configuration
7. 7 If module is dead, swap with a spare — the terminal base retains all field wiring for fast swap

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