SCALANCE X204-2 Red Fault LED: Diagnosing PROFINET IO Errors

1. Problem Definition

A Siemens SCALANCE X204-2 managed industrial Ethernet switch exhibits a continuously illuminated red "F" (Fault) LED while all port activity LEDs (yellow/green for link and traffic) continue to flash normally. Layer 2 traffic is unaffected, and the switch continues to forward frames at wire speed. However, the lit Fault LED also drives the integrated signaling contact to the open (fault) state, which supervisory systems, SCADA, or HMI panels may interpret as a device-level alarm.

This condition is commonly observed in plants where:

• A PROFINET IO controller (typically a SIMATIC S7 CPU, ET 200 station, or third-party PN controller) has previously assigned the SCALANCE X204-2 a PROFINET IO device name.
• The PROFINET IO controller is no longer reachable (controller offline, wrong subnet, removed, or reconfigured).
• Port-level fault masking (Link Down monitoring) has been explicitly disabled.
• Redundant power supply monitoring is disabled, eliminating that fault path.

2. SCALANCE X204-2 Hardware Reference

The SCALANCE X204-2 (Siemens order number 6GK5204-2BB10-2AA3 for the standard version, with variants for conformal coating and different approvals) is a managed Layer 2 switch designed for PROFINET and standard Ethernet networks in industrial environments. The relevant physical and logical inventory for fault diagnostics is:

For complete hardware and firmware specifications, consult the official Siemens operating instructions in the SCALANCE X-200 product manual entry on the Siemens Industry Online Support portal.

3. Fault LED and Port LED Truth Table

The SCALANCE X204-2 front panel exposes one Fault LED (red, labeled "F"), one or two status LEDs per port (green for link, yellow for activity), and a power LED. The combinations in the table below are taken directly from the device manual:

4. Documented Fault Sources

According to the SCALANCE X-200 manual, the Fault LED is driven by an internal fault state machine. The four documented fault sources are:

1. Link Down event on a monitored port — Only triggers the Fault LED if the corresponding port has Link Monitoring enabled in the fault mask. If you have explicitly cleared all fault masks, this source cannot be the cause.
2. Failure of one of the two redundant power supplies — Only triggers the Fault LED if redundant power supply monitoring is enabled. If this feature is disabled (as is common for cost-optimized single-supply installations), this source cannot be the cause.
3. C-PLUG error — The removable C-PLUG stores the device configuration. A fault is raised only when an inserted C-PLUG is empty, corrupt, mismatched to the firmware, or unreadable. If the switch is running with the internal backup configuration and no C-PLUG present, no fault is raised.
4. PROFINET IO error — The switch has been assigned a PROFINET IO device name (PN IO Name) and is expected to participate in PROFINET IO communication, but no PROFINET IO controller has established an AR (Application Relationship) with the device. This is the most common cause of a persistent red Fault LED with otherwise normal switching behavior.

5. Root Cause: PROFINET IO Device Name Without an Active IO Controller

When a PROFINET IO controller commissions a PROFINET device, it assigns a unique PROFINET IO device name to that device using the DCP (Discovery and Configuration Protocol) set-name service. Once the name is written, the device expects to be discovered by an IO controller under that name and to establish an AR (Application Relationship).

Typical scenarios that leave a stray PN IO Name in the SCALANCE:

• The PLC was replaced or reconfigured and the old PN IO Name is no longer assigned to any active controller.
• The SCALANCE was moved to a different subnet/VLAN, leaving it addressable by IP but invisible to DCP from the IO controller.
• A configuration tool (STEP 7, TIA Portal, Primary Setup Tool, PRONETA) wrote a temporary name to the switch during commissioning and never cleared it.
• The switch was inherited from a decommissioned machine and re-purposed without a factory reset.

The switch retains this PN IO Name in non-volatile storage (internal flash or C-PLUG). Every time it boots, it announces the name and waits for an AR. When none is established, the internal fault state machine asserts the "PROFINET IO Error" fault, lighting the F LED and opening the signaling contact.

6. Diagnostic Procedure

Before applying any remedy, confirm the active fault source. Three methods are available, listed in order of preference:

1. Read the fault via Web Based Management (WBM) — Connect a service laptop to any switch port, browse to the device's IP address, and inspect the Information > Fault Status or System > Events page. The active fault is named explicitly (for example, "Fault: PROFINET IO Error").
2. Read the fault via CLI (SSH or Telnet) — Log in and run the command show fault or info (depending on firmware) to retrieve the active fault mask and current state.
3. Read the fault via SNMP — Poll the SCALANCE private MIB. A fault-state trap is raised on every fault state change; the corresponding OID is in the Siemens enterprise branch 1.3.6.1.4.1.4329. See the SCALANCE X-200 manual for the exact OID table.

For a switch with no IP address (out-of-the-box or after a config reset), assign a temporary IP using the Siemens Primary Setup Tool (PST) or PRONETA, accessible only on the same Layer 2 segment. Do not assign an IP that is already in use on the production network — use a dedicated service address such as 192.168.0.250/24 for the duration of the diagnostic session, then release it.

7. Solution: Delete the PROFINET IO Device Name via WBM

This is the targeted, non-destructive remedy that clears the fault without disturbing the rest of the running configuration (VLANs, ring ports, SNMP communities, ACLs, fault masks, RSTP priorities).

7.1 Prerequisites

• Known IP address of the SCALANCE X204-2 (or a temporary IP assigned via PST/PRONETA).
• Admin credentials for the WBM. Default username is admin; the default password is the device's serial number or admin depending on firmware version.
• Service laptop on the same subnet or directly connected to any port (the SCALANCE auto-MDI/MDIX handles both crossover and straight-through cables).

7.2 Procedure

1. Open a modern browser (Chrome, Firefox, Edge). Disable proxy interception if present.
2. Navigate to http://<switch-IP>. Accept the self-signed certificate warning.
3. Log in with administrator credentials.
4. In the left navigation tree, expand Information > PROFINET IO (path varies slightly between firmware releases). Confirm that a non-empty "PROFINET IO Device Name" is displayed.
5. Open System > PROFINET IO (or Layer 2 > PROFINET in newer firmware) and locate the input field for the device name.
6. Delete the contents of the Device Name field so that it is empty. If the firmware offers an explicit "Disable PROFINET IO device role" checkbox, enable that as well.
7. Click Set Values or Apply. The change is committed to the active configuration.
8. Navigate to Information > Fault Status and verify that the "PROFINET IO Error" entry has cleared. The Fault LED on the device should extinguish within a few seconds.
9. Save the configuration with System > Save (or equivalent) to persist the change across reboots.

7.3 CLI Equivalent

For users who prefer the command line, the equivalent procedure over SSH/Telnet is:

enable
configure terminal
no pn-io device-name
pn-io station-name ""
end
write memory
show fault

The exact command syntax varies between firmware versions. Consult the SCALANCE X-200 Command Line Interface reference in the Siemens Industry Online Support for the firmware-specific syntax tree.

8. Alternative: Restore Factory Defaults

If WBM access is unavailable and the CLI cannot be reached, restoring factory defaults will also clear the PROFINET IO Name. The trade-off is that all other configuration parameters — ring ports, VLANs, SNMPv3 users, IP ACLs, port mirroring, fault masks, RSTP priorities — are lost and must be re-entered.

Two methods are supported:

• WBM: System > Reset > Restore Factory Defaults and Restart. The switch reboots; allow 60–90 seconds for full initialization.
• Hardware button: Some SCALANCE X-200 variants expose a recessed reset button on the front panel. Hold it during power-up until the Fault LED blinks, then release. The switch boots with factory defaults.

9. C-PLUG Considerations

If the WBM shows a "C-PLUG Error" rather than a PROFINET IO error, the procedure differs:

• No C-PLUG inserted: Normal operation. The switch runs on the internal configuration memory. No fault.
• C-PLUG inserted but unreadable: Firmware mismatch or corrupt data. Remove the C-PLUG, reinsert it, and confirm in WBM under System > C-PLUG.
• Format the C-PLUG: From WBM, System > C-PLUG > Format writes a fresh empty configuration onto the plug. The switch then starts with its internal backup configuration.
• Replace the C-PLUG: If formatting fails, replace the C-PLUG with a new one (Siemens part 6GK1900-0AB00 for the SCALANCE X-200 family).

10. Signaling Contact Wiring and SCADA Integration

The SCALANCE X204-2 exposes a floating relay contact (signaling contact or "fault contact") on its front-panel terminal block. In normal operation the contact is closed; on any active fault it opens. The contact is rated for low-voltage signal use; consult the device manual for the exact current and voltage ratings before wiring it to a PLC digital input, a relay coil, or a SCADA RTU.

For PROFINET-based monitoring, the recommended pattern is to read the fault status via PROFINET IO diagnostics rather than wire the contact. In STEP 7 or TIA Portal, the SCALANCE PN IO device exposes the fault as a diagnostic alarm and as a status bit in the cyclic I/O image, which is more granular than the single-wire signaling contact.

11. Verification Steps

After applying the remedy, validate that the fault has cleared end-to-end:

1. Physically observe the SCALANCE front panel: the red "F" LED should extinguish within 5–10 seconds of the WBM/CLI change.
2. Confirm in WBM: Information > Fault Status should show "No fault" or an empty list.
3. Measure the signaling contact: the resistance across the contact terminals should read near zero (closed) when measured with a multimeter in resistance mode.
4. If the SCALANCE is supervised by a PROFINET controller, check the controller's online diagnostics for the device. The diagnostic alarm buffer should no longer contain "PROFINET IO Error" entries after the next DCP discovery cycle.
5. If the SCALANCE is supervised by SNMP, poll the fault-state OID to confirm a "no fault" value. Clear the trap log on the NMS to avoid spurious alarms from the prior fault events.
6. Reboot the SCALANCE once after clearing the name to confirm that the fault does not return at startup — this validates that the change has been written to non-volatile storage.

12. Preventive Best Practices

• Clear PN IO Names from decommissioned or temporarily used SCALANCE switches as part of the removal procedure, before the switch leaves the subnet.
• Maintain a naming convention (for example sw-line3-cellA) and document which IO controller owns which switch name. Tools like Siemens PRONETA can scan a network and report all assigned PN IO Names in one pass.
• Avoid commissioning tools that write a temporary PN IO Name unless the tool also clears it on exit.
• Configure the SCALANCE fault mask deliberately. Link monitoring should be enabled only on ports where a link-down is genuinely abnormal; disable it on uplink ports, ring ports, and ports to mobile equipment.
• If the Fault LED is wired to a higher-level alarm system, add a "PROFINET IO Error" filter so that an isolated PN IO name does not trigger a top-level plant alarm when the switching function is healthy.
• Schedule a periodic PRONETA network scan as part of preventive maintenance to detect orphaned PN IO Names before they generate nuisance alarms.

FAQ