A consolidated reference for the Mitsubishi FX5U faults you are most likely to hit — CPU LED states, special relay flags and Ethernet errors — with the common remote-checkable cause for each and the diagnosis to run before you consider swapping the CPU.
The FX5U signals a fault through its front-panel LEDs (RUN, ERROR, and the I/O/module states) and through the special relays and registers (such as SM/SD) that latched the event. GX Works3 diagnostics decode the specific error code from the CPU. Reading the LED pattern and the diagnostics together is what a remote specialist does to isolate the problem.
| Fault source | Where you see it | What it tells you |
|---|---|---|
| CPU LEDs | FX5U front panel | RUN/STOP and ERROR states |
| Special relays | SM flag in GX Works3 / ladder | Latched error flags the program can read |
| Diagnostics | GX Works3 Diagnostics | The error code, module and time of the fault |
The table lists the most common FX5U fault classes, their meaning, and the first remote check. Likelihood reflects field frequency.
| Fault / Code | Meaning | Likelihood | First Remote Check |
|---|---|---|---|
| ERROR LED on | A CPU fault was detected | Med | Open GX Works3 diagnostics and read the error code and module |
| Ethernet / comm error | CPU lost its network partner | Med–High | Confirm the CPU IP/subnet and ping the partner from the same network |
| Special relay SM0/SM5+ | Latched system or software error flags | Med | Read the SM flags described in the FX5U programming manual for the exact class |
| Watchdog / scan stop | Task exceeded the scan time limit | Low–Med | Review the program scan time and any infinite loop or heavy task |
| Parameter error | Invalid or inconsistent CPU/module parameter | Low–Med | Review recently changed parameters and compare to the working backup |
| I/O module fault | Configured I/O or expansion module errored | Med | Read the module status in GX Works3 and check its power and connection |
The FX5U exposes the exact fault through its diagnostics and SM/SD flags, so the LED alone is only a starting point. Reading the diagnostics is what isolates the real trigger.
Work through these in order. Each step is something you can perform and report back on without opening the panel or removing parts.
Open Diagnostics in GX Works3 and note the error code, the module and the timestamp. This is the clearest starting point for remote analysis.
Photograph the RUN/ERROR LEDs and note the relevant SM flags. The pattern adds context to the diagnostics.
Was there a program download, a module add, a firmware update or a wiring change right before? Recent changes usually reveal the trigger.
Confirm the online parameters match the working backup and the module firmware is compatible. A config mismatch is a common one-shot fault trigger.
After clearing the cause, reset the fault and confirm the CPU runs and the network partner stays connected for a sustained period.
An FX5U fault is a communication, programming or module path more often than a CPU hardware failure. Cross-reference the specific conditions: Ethernet error · All controller symptoms · PLC technology.
In remote practice, most FX5U faults are communication, programming or I/O-module paths and are resolved from the ERROR LED, the special relay flags and the GX Works3 diagnostics — not by replacing the CPU. Escalate to a specialist when a hardware-flagged fault survives a full power cycle, when a bus or module error reproduces after reseating checks, or when production is down and a wrong hardware order would be costly.
To get the fastest accurate read, send these three things. All are taken from the LEDs and GX Works3 — no hardware removal.
Send these and we can usually confirm whether the fault is remote-resolvable before you book any hands-on visit. machinery controller troubleshooting guidance
The ERROR LED turns on when a CPU fault occurs. It can be a programming, hardware/system, or watchdog timeout error. The exact cause is read from GX Works3 diagnostics, not from the LED alone.
It means the CPU cannot establish or maintain a connection with its network partner. It is usually an IP/subnet, port, or cable path issue that is remotely-checkable from the CPU's network settings and the partner's response.
Yes. Share the LED states, the GX Works3 diagnostics and what changed; a specialist interprets the error and guides recovery. We never take control of your controller.
The FX5U flags a battery condition when the internal battery voltage is low or was interrupted during a power change. In many cases the battery is simply aging and should be replaced with the correct permanent-battery type — but the flag can also signal a deeper module or memory issue. A quick check of the SM flag and module diagnostics tells which one you are dealing with.
Send us the error code, a photo of the screen and what you already tried — an independent specialist reviews your case and replies with an evaluation, typically within 24 hours.
Submit Your CaseSend the diagnostics, LED state and what changed. We interpret it and guide recovery remotely — no CPU swap unless it is genuinely required.
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.
Before changing drive parameters or replacing parts, verify against the official documentation from Mitsubishi Electric:
Tip: for simple issues, a general-purpose AI assistant can help you interpret a fault code — but always cross-check any corrective action against the official manual before applying it.
The most common network trip path in detail.
A related PLC fault reference for another vendor.
A complete PLC fault-code reference.
Full controller troubleshooting collection.
Part of the full library: fault codes by brand — ACS580, S7-1200, ATV320, Sigma-7, CompactLogix, FX5U, VF-2, Robodrill and CFW800.