Why Is My PLC Losing Memory or Resetting?

Program disappearing after power cycle

PLC resets or memory loss typically show up as:

Program disappearing after power cycle
Machine returning to factory defaults
Random restarts during production
Time/date resetting
I/O mapping lost
Unexpected fault messages

This is almost never a “software bug.”
It is almost always power or hardware related.

1. PLC Backup Battery Failure (Most Common Cause)

Most PLCs use an internal battery to:

Retain program memory
Preserve parameters
Maintain clock and data

If battery fails:

Program may erase after shutdown
Time/date resets
Parameters disappear

What to Check:

✔ Look for “Low Battery” warning
✔ Check battery replacement interval
✔ Replace with manufacturer-specified battery

Never ignore battery warning alarms.

2. Unstable Power Supply (Very Common in Industrial Sites)

PLC power supply instability can cause:

Random resets
Memory corruption
Partial boot failures

Causes:

Voltage drops
Phase imbalance
Loose wiring
Overloaded control transformer

What to Check:

✔ Measure 24V DC stability
✔ Inspect power supply terminals
✔ Check incoming AC voltage under load
✔ Verify proper grounding

PLC requires stable, clean voltage.

3. 24V Power Supply Failure or Undersizing

If the 24V power supply is:

Undersized
Overloaded
Aging

It may drop voltage during high demand (solenoids, relays switching).

Symptoms:

PLC resets when hydraulic solenoids activate
Reset during punch or shear cycle

Fix:

✔ Measure voltage during peak load
✔ Upgrade to properly sized power supply
✔ Separate PLC power from heavy solenoid loads

Control voltage must be isolated from high-current devices.

4. Electrical Noise & Interference (VFD / Motor Noise)

High-frequency noise from:

VFDs
Motors
Large solenoids

Can cause:

PLC communication faults
Unexpected resets
Data corruption

Fix:

✔ Ensure proper grounding
✔ Separate signal and power cables
✔ Use shielded cables
✔ Install ferrite cores if necessary

Noise is a common hidden cause of instability.

5. Corrupt Memory or Flash Wear

PLC memory can degrade over many years.

Especially if:

Frequent program downloads
Power interruptions during writes
Poor shutdown practices

Signs:

Random loss of specific parameters
Program partially corrupted

Fix:

✔ Reload clean backup
✔ Update firmware
✔ Replace PLC if memory chip failing

Always keep a verified backup copy.

6. Loose Internal Module or I/O Rack Issue

In modular PLC systems:

I/O cards may loosen from vibration
Backplane connection may be unstable

Symptoms:

Intermittent resets
Communication errors
Partial I/O loss

Fix:

✔ Power down safely
✔ Reseat modules
✔ Inspect connectors

Vibration is common in roll forming environments.

7. Improper Shutdown Procedure

If power is cut abruptly during:

Data write
Parameter change
Firmware update

Memory may corrupt.

Use proper shutdown process whenever possible.

8. Overheating Control Cabinet

Excess heat inside electrical cabinet can:

Cause PLC reboot
Shorten battery life
Corrupt memory

Check:

✔ Cabinet fans
✔ Air filters
✔ Ambient temperature
✔ Dust buildup

Control systems need clean airflow.

9. Grounding Problems

Improper grounding can cause:

Floating voltage
Noise feedback
PLC instability

Verify:

✔ Solid earth grounding
✔ No ground loops
✔ Clean bonding between panels

Grounding is critical for PLC reliability.

10. Pattern Recognition

Symptom	Likely Cause
Loses program after power off	Backup battery dead
Resets during punching	24V voltage drop
Random resets at high speed	Electrical noise
Date/time always resets	Battery failure
Resets after long run	Overheating

Understanding the pattern dramatically narrows the cause.

11. Step-by-Step Diagnostic Procedure

If PLC is resetting:

Check for battery alarm
Replace battery proactively
Measure 24V under load
Check grounding integrity
Inspect cabinet cooling
Review VFD and motor cable routing
Reload clean backup program
Monitor system over multiple runs

Never operate without a verified program backup.

12. Why PLC Resets Are Dangerous

PLC resets can cause:

Punch misfires
Shear misalignment
Unexpected machine motion
Production downtime
Parameter loss

Control stability is critical for safe operation.

Final Expert Insight

PLC memory loss or resets are usually caused by:

✔ Dead backup battery
✔ Power supply instability
✔ 24V voltage drop
✔ Electrical noise
✔ Overheating
✔ Loose I/O modules
✔ Corrupt memory

The most common cause is a failing backup battery combined with unstable control voltage.

A stable roll forming machine requires:

Clean power → Stable 24V supply → Good grounding → Proper cooling → Regular battery replacement.