Schneider Modicon M340 / M580 — ERR LED Fault Diagnostics

Schneider · intermediate · 30 min

An ERR LED on a Modicon M340 or M580 CPU means a detected fault. Read the CPU status words and use EcoStruxure Control Expert's Diagnostic Viewer to find the exact cause before you reset.

ℹ️ The Modicon CPU exposes its fault state in system words you can read live. Reading %SW10 (M340) or %SW33 (M580) plus the Diagnostic Viewer beats swapping modules blind.

What the ERR LED means

On a Schneider Modicon M340 (BMX P34) or M580 (BME P58) CPU, the ERR LED signals a detected fault — a hardware problem, an I/O fault, a configuration mismatch, or an application error. The RUN, ERR, and I/O LEDs together give a first indication; the exact cause comes from the CPU's system words and EcoStruxure Control Expert (formerly Unity Pro).

Read the CPU status

  1. Check the LEDs. Note the state of RUN, ERR, and I/O on the CPU and any flagged module. Blink patterns map to fault classes in the M340/M580 hardware manual.
  2. Read the fault system words. Go online in Control Expert and read:
  • %SW10 on M340 — a non-zero value indicates a hardware fault (each bit flags a detected fault condition).
  • %SW33 on M580 — same idea for the M580 CPU.

A non-zero value tells you a hardware/self-test fault is present rather than a pure application issue.

  1. Use the Diagnostic Viewer. In EcoStruxure Control Expert online mode, open the Diagnostic Viewer for detailed, timestamped error information, and confirm each module is correctly configured in the project versus the physical rack.

Resolve by cause

  • I/O module fault: confirm the module receives correct voltage/current, check for a blown fuse or tripped breaker, and make sure the module is fully seated in the rack/backplane. Verify its configuration matches the project.
  • Configuration mismatch: if the physical rack no longer matches the Control Expert project, correct and re-download the configuration.
  • Application fault: use the Diagnostic Viewer and program diagnostics to find the offending logic or data condition.

Reset

Once the cause is corrected, clear the fault. Pressing the CPS (power supply) reset button causes the CPU to perform a Cold Start; whether data and the RUN/STOP condition are retained depends on your configuration settings — know your project's cold-start behavior before pressing it, since it can clear retained data. For many faults, correcting the cause and switching the CPU back to RUN in Control Expert is sufficient.

Verify

After the fix, confirm the ERR LED is off, RUN is on, and %SW10 / %SW33 reads zero. Re-open the Diagnostic Viewer to confirm no fault remains; a value that returns immediately means the hardware cause is still present.

Key points

  • RUN/ERR/I/O LEDs give the first indication; blink patterns map to fault classes in the hardware manual.
  • Read %SW10 (M340) or %SW33 (M580) online — a non-zero value indicates a hardware/self-test fault.
  • Use EcoStruxure Control Expert's Diagnostic Viewer for detailed, timestamped error information.
  • For I/O faults check supply voltage, fuses/breakers, and that the module is fully seated; verify config matches the project.
  • The CPS reset button forces a Cold Start — retention of data and RUN/STOP depends on configuration, so know the behavior first.

Codes and symptoms

ERR LED — Detected fault
The CPU flagged a hardware, I/O, configuration, or application fault; read %SW10/%SW33 and the Diagnostic Viewer for the cause.
%SW10 / %SW33 — CPU fault status word
Non-zero indicates a detected hardware fault on M340 (%SW10) or M580 (%SW33).