In this guide
A controller that will not enter RUN is stressful because everything stops. A calm, ordered check finds the cause faster than swapping parts. This is a generic first pass that applies to most brands, and the specifics of LEDs and software differ between them.
Treat the panel as live. Follow your site's lock-out procedure and have only qualified people work inside it.
1. Capture the evidence first
Before you power-cycle anything, photograph the LEDs on the CPU and nearby modules. Many controllers lose some fault information when restarted.
2. Check power
Measure the 24 V supply at the PLC's own terminals, with the system running. A supply that reads 24 V unloaded can sag under load. Low voltage, noisy supply or a tripped branch breaker is a frequent cause of random stops.
3. Check the mode
- Is a key switch or mode switch set to STOP or PROG?
- Has someone put the CPU in STOP from the software?
4. Read the LEDs
Each brand labels them differently, but they generally indicate:
- A system or group fault on the CPU
- A bus or network fault
- A module fault or missing module
- A low or missing battery
Look up the exact meaning in the CPU's manual.
5. Read the diagnostics
Connect with the engineering software and read the diagnostic or fault log. It normally names the cause and the time it happened, for example a module that stopped responding, a program error or a watchdog timeout. Do this before changing hardware.
6. Look for what changed
Controllers often refuse to run after a replacement because the new part does not match the configuration. Ask:
- Was a module recently replaced with a different type or version?
- Was a CPU replaced, so the memory card or program no longer matches?
- Was firmware changed?
7. Check memory, card and battery
A missing or corrupt memory card, or a flat backup battery on older controllers, can stop a CPU from running or can wipe retained data. Check what your CPU uses and its condition.
8. Check communications and remote I/O
A missing remote I/O node or a broken network cable can stop a CPU that is set to stop on I/O failure. Look at the bus LEDs and reseat connectors before assuming the CPU is faulty.
9. Make sure you have a backup
If the CPU has failed and a replacement is on the way, you will need the program. Locate your latest backup now. If you do not have one, tell us at the start so we can plan the recovery.
What to send us
- The CPU's full order number and firmware version
- A photo of the LEDs and the fault information
- What changed recently
- Whether you have a program backup