Table of Contents

Introduction

The Mitsubishi FX3U series PLC is one of the most widely used compact programmable logic controllers in industrial automation. It is commonly deployed in packaging machinery, food processing equipment, conveyor systems, textile machines, water treatment plants, and factory automation applications.

Although the FX3U PLC is highly reliable, users may occasionally encounter CPU faults, communication errors, memory issues, power supply alarms, I/O abnormalities, and special module diagnostics.

Understanding these error codes is essential for minimizing downtime and improving system availability.

This guide provides a complete reference for Mitsubishi FX3U error codes, troubleshooting procedures, and maintenance recommendations.

Overview of Mitsubishi FX3U PLC

Mitsubishi FX3U Series PLC belongs to the third generation of Mitsubishi compact PLCs and offers:

High-speed processing
Flexible expansion capability
Built-in communication functions
Multiple special function modules
Compatibility with GX Developer and GX Works2

Typical models include:

FX3U-16MR/ES-A
FX3U-32MR/ES-A
FX3U-48MR/ES-A
FX3U-64MR/ES-A
FX3U-80MR/ES-A
FX3U-128MR/ES-A

Understanding FX3U Diagnostic LEDs

Before checking software error codes, engineers should inspect the PLC status LEDs.

LED	Status	Meaning
POWER	ON	Power supply normal
RUN	ON	CPU running normally
ERROR	OFF	No fault
ERROR	ON	System fault detected
BAT	ON	Battery low
SD	Flashing	Data communication active

Error Code 6100 – Program Check Error

Description

The PLC detects a syntax or instruction error during program verification.

Possible Causes

Invalid instruction
Incorrect operand
Corrupted project file
Unsupported instruction set

Troubleshooting

Open GX Works2.
Perform Program Check.
Identify highlighted instruction.
Correct syntax.
Download revised program.

Error Code 6101 – Device Range Error

Description

Specified device address exceeds allowable range.

Example

Incorrect:

D90000

Valid:

D0 ~ D7999

Solution

Verify:

Data registers
Timers
Counters
Internal relays

Error Code 6102 – Duplicate Coil Error

Description

Multiple outputs are assigned to the same device.

Example:

Y0

used repeatedly in several ladder sections.

Impact

Unpredictable machine operation
Output conflicts
Logic instability

Resolution

Use:

SET / RST instructions
Internal relays (M devices)

instead of duplicate coils.

Error Code 6200 – Parameter Error

Description

PLC parameter settings are invalid.

Causes

Wrong communication settings
Incorrect PLC type selection
Invalid I/O configuration

Solution

Verify:

PLC model
Communication parameters
Module settings

Then rewrite parameters.

Error Code 6201 – Memory Capacity Error

Description

User program exceeds available memory.

Typical Symptoms

Download failure
Program compile error
CPU enters STOP mode

Solutions

Optimize Program Structure

Reduce:

Duplicate code
Unused labels
Unused comments

Use Subroutines

Convert repetitive logic into:

CALL
FEND
RET

structures.

Error Code 6300 – Watchdog Timer Error

Description

Scan time exceeds allowable CPU limit.

Symptoms

CPU stop
Error LED illuminated

Causes

Infinite loop
Excessive FOR/NEXT operations
Communication overload

Troubleshooting Steps

Step 1

Check scan time monitor.

Step 2

Review loops.

Step 3

Reduce unnecessary calculations.

Step 4

Optimize communication polling.

Error Code 6400 – Battery Voltage Low

Description

Internal memory backup battery voltage is below specification.

LED Indication

BAT indicator turns ON.

Risks

Program loss
Data register loss
RTC reset

Recommended Action

Replace battery immediately.

Common battery model:

FX3U-32BL

Error Code 6401 – Battery Missing

Description

Battery disconnected or not installed.

Inspection

Check:

Battery connector
Wiring condition
Battery holder

Error Code 6500 – EEPROM Write Error

Description

PLC cannot save data to internal EEPROM.

Causes

Memory corruption
Power interruption during write process
Hardware failure

Solution

Backup project.
Clear memory.
Rewrite program.
Replace CPU if necessary.

Error Code 6600 – Communication Error

Description

PLC communication timeout detected.

Applicable Networks

RS232
RS485
Ethernet adapters
Modbus RTU

Common Causes

Wrong Baud Rate

Example:

PLC:

HMI:

Wrong Station Number

Cable Failure

Noise Interference

Recommended Checks

Communication parameters
Shield grounding
Connector integrity

Error Code 6700 – Intelligent Module Error

Description

Special function module reports abnormal status.

Affected modules may include:

Analog modules
Temperature modules
High-speed counter modules
Positioning modules

Diagnostic Procedure

Check module LEDs.
Verify module parameters.
Confirm wiring.
Read module buffer memory.

Error Code 6800 – I/O Configuration Error

Description

Installed hardware differs from configured hardware.

Example

Program expects:

FX3U-4AD

but module is absent.

Resolution

Update configuration and rewrite parameters.

Error Code 6900 – CPU Hardware Fault

Description

Internal CPU malfunction detected.

Symptoms

RUN LED OFF
ERROR LED ON
PLC not responding

Recommended Action

Power cycle PLC
Reload project
Replace CPU if fault remains

FX3U Communication Error Troubleshooting

Communication failures are among the most common field problems.

RS485 Troubleshooting Checklist

Wiring

Verify:

A+
B-
SG

connections.

Termination

Install:

120Ω

termination resistor.

Grounding

Use single-point grounding.

FX3U Analog Module Error Diagnostics

Frequently used modules:

FX3U-4AD
FX3U-4DA
FX3U-8AD

Common issues:

Input Overflow

Cause:

Input exceeds configured range.

Example:

20mA range configured

Actual signal:

24mA

Broken Sensor

Symptoms:

Input value fixed at maximum or minimum.

Check:

Sensor wiring
Power supply
Signal integrity

Preventive Maintenance Recommendations

To reduce FX3U failures:

Monthly

Inspect LEDs
Verify communication status

Quarterly

Check terminal tightening
Clean cabinet filters

Every 2 Years

Replace backup battery

Every 5 Years

Inspect power supply modules

Popular Mitsubishi FX3U Models

Model	Description
FX3U-16MR/ES-A	16 I/O PLC
FX3U-32MR/ES-A	32 I/O PLC
FX3U-48MR/ES-A	48 I/O PLC
FX3U-64MR/ES-A	64 I/O PLC
FX3U-80MR/ES-A	80 I/O PLC
FX3U-128MR/ES-A	128 I/O PLC
FX3U-4AD	Analog Input
FX3U-4DA	Analog Output
FX3U-ENET	Ethernet Module
FX3U-485ADP-MB	Modbus Adapter

Frequently Asked Questions

Why is the ERROR LED on but the PLC still running?

Minor faults such as battery alarms or communication warnings may not stop CPU execution.

Can I replace the battery without losing the program?

Yes. Keep the PLC powered during battery replacement whenever possible.

How do I read detailed error information?

Use GX Works2 Diagnostic Monitor or PLC Diagnostics.

What is the most common FX3U fault?

Communication errors, battery alarms, and parameter mismatches are the most frequently encountered issues.

Conclusion

The Mitsubishi FX3U PLC remains one of the most reliable compact controllers in industrial automation. Proper understanding of diagnostic LEDs, CPU fault codes, communication errors, memory alarms, and module diagnostics can significantly reduce machine downtime and maintenance costs.

Whether you are troubleshooting an FX3U CPU fault, communication timeout, battery warning, or I/O configuration issue, systematic diagnostics and preventive maintenance are the keys to reliable operation.

At PLC Provider, we not only offer technical support for Mitsubishi FX3U PLC troubleshooting, but also supply original new Mitsubishi PLCs, HMI panels, servo systems, inverter drives, communication modules, and industrial automation spare parts for global customers.