The FX5U loses communication over Ethernet — the GX Works3 link drops, a connected HMI or partner device stops exchanging data, or the built-in Ethernet LED shows no link.
These are the failure paths we see most often on the Mitsubishi FX5U for an Ethernet communication error condition. The FX5U's built-in Ethernet is almost always a config or link-layer issue, not a dead port. Likelihood reflects field frequency; First Check is what you can do remotely before sharing logs.
| Cause | Likelihood | First Check |
|---|---|---|
| IP / subnet mismatch with the partner device | High | Check the FX5U IP, subnet and gateway against the partner and the LAN |
| Wrong network parameters (protocol, port, open settings) | Medium | Check the Ethernet configuration in GX Works3 |
| Cable / switch / link-layer fault (no link LED) | Medium | Check the cable, switch port and the PHY link LED |
| Duplicate IP on the network | Low | Scan the subnet for a conflicting device |
Work through these in order. Each step is something you can perform and report back on without opening the enclosure or swapping parts.
Open the FX5U diagnostics and capture the Ethernet error or system-status entry, including any error code. That code is what we interpret.
Confirm the FX5U IP, subnet mask and gateway match the partner device and the LAN. A changed IP or a wrong octet is the usual trigger for an Ethernet error.
Confirm the protocol (MC/SLMP or socket) and the port number in the FX5U Ethernet parameter configuration. A mismatched port or an unopened connection drops the session.
Check the PHY/link LED and swap the cable and switch port to rule out a hardware link fault before touching the configuration.
Scan the subnet for a device holding the same address. A duplicate IP forces the session to drop and is easy to miss on a busy LAN.
An FX5U Ethernet error is a configuration or link-layer problem, rarely a dead port. The GX Works3 diagnostic plus a quick IP and link check isolate it: no link LED means cable or switch, a clean link but no session means IP or open-settings, and a flapping session means a duplicate IP. Cross-reference the broader collections: Communication / link loss · PLC & Ethernet.
When to escalate: if the link LED is dead after a cable and switch change, the PHY may be at fault — but we confirm that from the link test before suggesting a CPU. Most Ethernet errors we address are IP, port or open-setting mismatches that you correct in GX Works3.
Rarely. Ethernet errors are configuration or link-layer; we address them from the diagnostic. Hardware is only suspected if the PHY is dead, and we confirm that from the link LED first.
Usually an IP/subnet mismatch or an open-setting mismatch. We verify both from your configuration and tell you the exact value to correct.
No. You read the diagnostic in GX Works3 and relay it; we interpret it. We do not take control of your controller.
Possibly — a dead link LED points to cable or switch. We have you isolate that first before looking at the configuration.
Send the GX Works3 diagnostic and your IP config. We split config, link and duplicate-IP from the readings remotely so you avoid swapping a good CPU.
Submit Your Case Connect With SpecialistsWe do not perform hardware repair, we do not replace parts, and we cannot promise a 100% resolution. Our role is limited to remote diagnostic and step-by-step guidance based on the information, photos and logs you share.
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