PLC Restart Loop After Power Restoration | Causes, Diagnosis & Fixes

 Power outages are common in industrial environments, but when power returns, some PLCs don’t resume normal operation — instead, they enter a continuous restart loop. This condition disrupts automation, causes downtime, and sometimes damages connected devices.

If your system shows this symptom, the cause could range from unstable power to firmware corruption. In this guide, we’ll answer the most practical technical questions engineers ask when troubleshooting this issue.

1. Why Does the PLC Keep Restarting Continuously After Power Is Restored?

A PLC restart loop typically means the CPU repeatedly tries to initialize but encounters a fault that prevents it from reaching “RUN” mode.
Common root causes include:

• Unstable power during startup

• Faulty internal components or firmware

• Corrupted logic files or retentive memory errors

• Hardware modules that fail to respond during initialization

Each restart is often triggered when the PLC detects an internal fault condition, shuts down to protect itself, and then reattempts the boot sequence.

Read about: PLC Not Starting After Power Outage | Troubleshooting Guide

2. Could a Voltage Fluctuation or Power Surge Cause the Restart Loop?

Yes — voltage fluctuations and transient surges during power restoration are among the top causes.
When supply voltage rises above the PLC’s rated tolerance (usually ±10%), internal regulators shut down to prevent circuit damage. Similarly, a brief undervoltage (brownout) can cause the CPU to restart mid-initialization.

How to test:

• Use a true RMS multimeter or oscilloscope to monitor voltage levels during startup.

• Compare readings with the PLC power supply specifications (typically 24 VDC ±10%).

• If spikes occur, consider installing surge suppressors, line filters, or an industrial UPS for the control system.

3. How Do I Check if the PLC Power Supply Is Unstable or Faulty?

A weak or degraded power supply module can cause intermittent reboots even with stable mains voltage.
Symptoms include:

• Voltage drops during I/O activation

• Power LEDs flickering or dimming

• Unusual heat from the supply module

Steps to diagnose:

1. Measure DC output (e.g., 24VDC) under load using a multimeter.

2. Observe if the voltage dips below the lower threshold during startup.

3. Temporarily power the PLC from a separate, known-stable DC source.
   If the restarts stop, the power supply module is faulty and should be replaced.

4. Can a Damaged CPU or Corrupted Firmware Trigger Repeated Restarts?

Absolutely. The CPU module contains both hardware and firmware responsible for initializing the system.
If firmware corruption occurs (due to power loss during write operations), the PLC may enter a continuous reboot cycle.

Symptoms:

• CPU fault LED remains on or flashes a repeating pattern.

• No communication via programming software.

Action:

• Connect via programming cable and check diagnostic status.

• If inaccessible, reload or re-flash the firmware using the vendor’s utility (e.g., Siemens TIA Portal, Allen-Bradley ControlFlash, or ABB Automation Builder).

• If firmware recovery fails, the CPU may need replacement.

5. How Can I Tell if the PLC Program Got Corrupted After the Outage?

Corrupted user logic is a classic result of sudden power loss. If the memory didn’t complete a write operation, the logic files might be unreadable or inconsistent.

Check for:

• Error messages like “Invalid program checksum” or “Project mismatch.”

• PLC boots into STOP mode and doesn’t switch to RUN even after manual attempt.

Fix:

• Reload the last known working program from backup.

• Validate the checksum or project version using the engineering tool.

• Always keep a verified backup on external storage or within SCADA.

6. Do Memory Retention or Battery Failures Lead to Restart Loops?

Yes. Most PLCs rely on a backup battery or supercapacitor to retain data (counters, timers, memory bits) during power loss.
If the retention battery is drained, the PLC may try to restore lost memory on each boot and fail — causing repeated restarts.

Check:

• Battery status indicator or diagnostic log.

• Measure the voltage (typically 3V coin cell or lithium pack).

• Replace the battery and reload configuration.

Preventive step: replace retention batteries every 2–3 years as per manufacturer recommendations.

7. Could Poor Grounding or Electrical Noise Be Causing PLC Resets?

Definitely. Electrical noise or improper grounding induces transient voltages that can reset CPU logic circuits.
Noisy environments (with VFDs, welders, or heavy motors) often create induced voltage spikes in PLC wiring.

What to do:

• Ensure separate grounding for signal and power circuits.

• Verify shield continuity for analog and communication cables.

• Add ferrite cores or filters on communication lines.
  Proper earthing and EMI suppression can eliminate phantom restarts that appear random.

8. How Do I Verify if the Input Voltage and Grounding Are Within Limits?

For accurate assessment:

1. Measure input voltage at the PLC power terminals while energizing I/O.

2. Confirm that ground potential difference between control cabinet and earth ≤ 1V.

3. Use a megohmmeter to test insulation resistance of wiring.

4. Inspect for corrosion or loose ground screws.

Consistent grounding and stable input voltage are essential to PLC reliability after power events.

9. Is It Possible That a Faulty I/O Module or Backplane Causes Reboots?

Yes — a defective I/O module or damaged backplane can draw excess current or fail to initialize, forcing the CPU to reset.
Each restart occurs as the CPU polls the module, detects fault, and restarts to retry initialization.

Diagnostic steps:

• Remove non-essential I/O modules one by one and power on.

• If the restart stops after removing a certain module, replace it.

• Inspect the backplane connectors for oxidation or bent pins.

10. Can High Inrush Current During Power-Up Affect PLC Stability?

High inrush current (especially when powering multiple devices simultaneously) can drop the voltage momentarily and cause the PLC to reboot.
Large transformers, heaters, or VFDs starting at the same time can overload the circuit.

Preventive actions:

• Sequence device startup (PLC first, heavy loads later).

• Use soft-start contactors or current limiters.

• Increase power supply capacity or use dedicated control power.

11. Should I Check for Communication Overloads with HMI or SCADA After Restart?

Yes. During startup, HMI and SCADA systems flood the PLC with data requests.
If the PLC CPU is still initializing, this communication load can cause watchdog timeouts and force restarts.

Solution:

• Delay SCADA/HMI communication for a few seconds after PLC power-up.

• Use network-managed switches or software timers to stagger connections.

• Monitor communication logs for overload messages or retries.

12. Does a Watchdog Timer or Internal Fault Cause the PLC to Reboot Automatically?

Yes. The watchdog timer ensures that CPU cycles complete in expected time.
If the logic scan overruns or hangs, the watchdog forces an automatic restart.

Fix:

• Simplify long or nested loops in the PLC program.

• Reduce communication or instruction overhead.

• Check system diagnostics for “watchdog timeout” entries.

13. How Can I Access Diagnostic Logs or Fault Buffers to Find the Root Cause?

Each PLC brand provides diagnostic tools:

• Siemens: Diagnostic Buffer in TIA Portal

• Allen-Bradley: Fault Log via RSLogix / Studio 5000

• Schneider: Event Log via Unity Pro or EcoStruxure

Check for recurring errors before restart, such as:

• “Power supply undervoltage”

• “Module missing”

• “Internal CPU fault”
  These records often pinpoint the restart trigger precisely.

14. Are Firmware Updates Necessary to Fix Repeated Restarts?

In many cases, yes. Manufacturers periodically release firmware updates to fix bugs related to power cycling and memory management.
Outdated firmware may mis-handle cold-start routines or fault recovery.

Best practice:

• Always match firmware with engineering tool version.

• Follow vendor instructions carefully when flashing firmware — avoid power loss during update.

15. What’s the Correct Power-Up Sequence for PLC and Connected Devices?

To prevent simultaneous current surges and data overloads:

1. Power on PLC power supply first.

2. Wait 5–10 seconds before energizing I/O and communication devices.

3. Finally, start VFDs, HMIs, and field instruments.

This ensures that the PLC completes initialization before field devices start sending signals.

16. Could an External Relay or Contactor Feedback Loop Trigger Restarts?

Yes. Incorrect wiring or feedback from contactor coils can backfeed voltage into PLC inputs or power lines, momentarily resetting the system.

Check for:

• Parallel relay wiring or back-EMF without flyback diodes.

• Shared power circuits between PLC logic and load devices.
  Solution:

• Add flyback diodes or RC snubbers to relay coils.

• Isolate PLC power from heavy inductive loads.

 Conclusion

A PLC restart loop after power restoration is rarely caused by one issue — it’s usually a combination of unstable power, firmware corruption, or hardware faults amplified by grounding or noise.

A structured troubleshooting process should start from power quality, move through hardware integrity, then verify firmware and memory, and finally check communication and sequencing.

Regular preventive maintenance, stable 24VDC supplies, good grounding, and up-to-date firmware are your best defense against these restarts.