Mitsubishi MELSEC ERR LED — Module Diagnostics in GX Works3

Mitsubishi · beginner · 25 min

A lit or flashing ERR LED on a MELSEC FX5U or iQ-R CPU means a diagnosable fault. Use GX Works3 Module Diagnostics to read the exact error code and clear continuation errors without a power cycle.

ℹ️ The ERR LED alone can't tell you whether it's a missing project, a bad parameter, a watchdog timeout, or a failed module. Every ERR LED should be followed by a GX Works3 diagnostic check.

What the ERR LED means

On a Mitsubishi MELSEC iQ-F (FX5U) or iQ-R CPU, the ERR LED lights or flashes when the CPU detects an error. MELSEC splits errors into two behaviors that decide your recovery:

  • Continuation errors — the CPU keeps running; something is wrong but not fatal. These can be cleared while the CPU stays powered, after you fix the cause.
  • Stop errors — the CPU stops operation. These require you to correct the cause and, depending on the error, reset or power-cycle.

The LED by itself doesn't distinguish these — GX Works3 does.

Read the error in GX Works3

  1. Connect the engineering PC to the CPU over USB or Ethernet and go online.
  2. Open Diagnostics → System Monitor to see the module layout and which module is flagged.
  3. Open Module Diagnostics for the flagged CPU or module. The Error Information tab shows the exact error code, its detailed cause, and the corrective action. Always read the detailed text — related codes can share an LED behavior but have different causes.

Clear the error

  1. Fix the cause first. Typical causes: a parameter/hardware mismatch, a missing or invalid project, a network/module connection loss, or a channel diagnostic on an I/O module with diagnostics. Resolve the specific item the Error Information tab names.
  2. Clear a continuation error. With the cause removed, clear it from the Module Diagnostics window. It can also be batch-cleared programmatically via special relay SM50 where your application uses that method.
  3. Recover a stop error. After correcting the cause, follow the corrective action shown — some stop errors clear on reset, others (parameter or hardware) require re-download of the correct project/parameters and a power cycle.

Verify

After clearing, confirm the ERR LED is off and the P.RUN / RUN LED indicates normal run. Re-open Module Diagnostics to confirm no error remains — if the same code returns immediately, its cause is still present.

Good practice

Enable diagnostic functions on I/O modules that support them so channel faults (wire break, short, over-range) appear in Module Diagnostics with a specific channel, and keep the corrective-action text from GX Works3 with your maintenance notes for recurring codes.

Key points

  • MELSEC errors are either continuation (CPU keeps running, clearable live) or stop (CPU halts) — the LED alone doesn't say which.
  • Go online with GX Works3, then use Diagnostics → System Monitor and Module Diagnostics to read the exact error code.
  • The Error Information tab gives the code's detailed cause and corrective action — always read it, related codes differ.
  • Continuation errors clear from Module Diagnostics (or batch via special relay SM50) once the cause is removed.
  • Stop errors need the cause corrected first; some clear on reset, parameter/hardware ones need re-download and power cycle.

Codes and symptoms

ERR LED — CPU error indicator
The CPU detected an error; read the exact code in GX Works3 Module Diagnostics to classify and clear it.
Continuation error — Non-fatal error
CPU keeps running; clearable from Module Diagnostics (or via SM50) after the cause is fixed.
Stop error — Fatal error
CPU stops; correct the cause, then reset or re-download and power-cycle per the corrective action.