ABB AC500 — ERR LED Diagnosis & Clearing (diagack)

ABB · intermediate · 25 min

An ABB AC500 ERR LED carries a class-and-number error code you read in Automation Builder. Find the flagged module, fix the cause, then clear with 'diagack all' in the PLC Browser or the Diag/OK buttons.

ℹ️ AC500 error codes are structured: a letter for the class and a number for the detail (e.g. E3/026). Read the code, go to the flagged module, then acknowledge — don't just power-cycle.

What the ERR LED means

On an ABB AC500 PLC, the ERR LED lights when the CPU registers a diagnosis entry. AC500 error codes use a class letter + number format — for example E3 is the class (a minor error) and the number (e.g. 026) is the detail (here, config data). Higher classes are more severe. The code tells you both how serious it is and where to look.

Read the error in Automation Builder

  1. Connect online with Automation Builder.
  2. Read the diagnosis. The CPU can display error codes on its display, and Automation Builder shows the diagnosis detail. For a module-related error (e.g. an E3 raised because a configured module has a fault), navigate online to the module that has the error and examine its diagnostic information — that pinpoints the failing I/O module and channel.
  3. Check the configuration match. Many AC500 faults are a mismatch between the hardware configuration in Automation Builder and the physical I/O. Use the 'Configure I/O' function to scan and verify the actual module layout, and make sure the project matches before downloading.

Clear the diagnosis

Once the cause is fixed, acknowledge the diagnosis by either method:

  1. PLC Browser method: in Automation Builder, open the PLC Browser (PLC Configuration tab) and type diagack all to clear all errors.
  2. CPU button method: push the 'Diag' button then the 'OK' button in succession until the faults are acknowledged.

Verify

After clearing, check the CPU status LEDs: RUN should be green and ERR should be off. If ERR comes back on, the underlying diagnosis is still active — re-read the code and go back to the flagged module. Acknowledging (diagack) clears the entry, but a real, still-present fault (a module fault, config mismatch, or wiring problem) will re-raise it.

Step-by-step

  1. Read the code (class letter + number) on the CPU display / Automation Builder.
  2. Go online to the flagged module and read its diagnostic info; for a config error, run Configure I/O and align the project to the physical layout.
  3. Fix the cause — module fault, config mismatch, or wiring.
  4. Acknowledge with diagack all in the PLC Browser, or Diag→OK on the CPU.
  5. Confirm RUN green, ERR off.

Good practice

Keep the Automation Builder project's hardware configuration exactly matched to the physical rack and re-run Configure I/O after any module change, so a config-data error (like E3/026) doesn't reappear after every download.

Key points

  • AC500 codes are class+number (e.g. E3/026): E3 = minor error class, the number is the detail (026 = config data).
  • Connect with Automation Builder, read the diagnosis, and for a module error navigate online to that module's diagnostic info.
  • Many faults are a hardware-config mismatch — use 'Configure I/O' to scan and align the project to the physical modules.
  • Clear with 'diagack all' in the PLC Browser, or push Diag then OK on the CPU until faults are acknowledged.
  • Verify RUN green and ERR off; a real fault re-raises after acknowledgment, so fix the cause first.

Codes and symptoms

E3 (class) — Minor error
A minor-class AC500 error; the accompanying number gives the detail (e.g. 026 = config data).
ERR LED — Diagnosis active
A diagnosis entry is present; read the class/number in Automation Builder and clear with diagack after fixing the cause.