Phoenix Contact PLCnext AXC — RUN/ERR/DIAG LEDs

Phoenix Contact · intermediate · 25 min

A Phoenix Contact PLCnext AXC F controller tells you its state on three LEDs — RUN, ERR and DIAG — before you ever open a laptop. Learn what each colour means and how to pull the Output.log for the real root cause of a DIAG warning or error.

ℹ️ On a PLCnext AXC F controller, read the RUN, ERR and DIAG LEDs first. DIAG amber/red is an actionable diagnostic event — don't ignore it, and capture the Output.log before you power-cycle, or you lose the evidence.

Read the three status LEDs

Phoenix Contact PLCnext AXC F controllers (AXC F 1152, 2152, 1050…) give you a quick local diagnosis on three LEDs:

• RUN (LED B) — green steady = the PLC is running; green flashing = the PLC is stopped.
• ERR (LED C) — red = a PLC fault; off = no fault.
• DIAG (LED D) — green = diagnostics OK; amber/orange = a warning event; red = an error; off = no diagnostic activity.

Between them you can tell at a glance whether the controller is running, faulted, or flagging a diagnostic condition — before connecting any software.

Treat DIAG amber/red as actionable

The DIAG LED's amber/red state should be treated as an actionable diagnostic event and not ignored. It's the controller telling you something in the runtime, the network, or the I/O needs attention. The mistake to avoid: power-cycling to "clear" it, which erases the evidence you need.

Capture the Output.log before power-cycling

The single most useful habit on PLCnext: capture the Output.log file before you power-cycle the controller. The runtime log lives at:

/opt/plcnext/logs/Output.log

Retrieve it from the controller over SCP. When you read it, focus on networking, RTEthernet, and DiagState entries — those confirm the root cause of most AXC F issues (for example a dropped RT Ethernet / dual-network connection). Power-cycling first throws this away; grab it first.

Step-by-step

1. Read RUN/ERR/DIAG. Running or stopped? Faulted? Diagnostic warning/error?
2. RUN flashing green (stopped)? The PLC isn't running — check why it stopped (a stop request, a startup/config error) rather than just restarting.
3. ERR red? A PLC fault — this pairs with a DIAG code; go to the log.
4. DIAG amber/red? Before anything else, SCP the Output.log off the controller.
5. Search the log for networking / RTEthernet / DiagState entries to find the root cause, then fix it (network wiring/config, I/O, or program) and restart.

Never do this

Don't power-cycle a PLCnext controller to make a DIAG warning go away before capturing Output.log — you'll destroy the one artefact that explains the fault, and it'll just come back. Pull the log, find the cause, fix it.