Top Mitsubishi PLC Error Codes Explained & How To Fix

Mitsubishi PLC systems such as FX3U / FX5U (iQ-F), Q Series, and iQ-R are widely used in CNC machines, automation cells, pallet systems, and auxiliary equipment. When faults occur, they are typically displayed as numeric error codes in GX Developer / GX Works, on the HMI, or via CPU LED indicators (ERR, RUN, BATT).

How to Read Mitsubishi PLC Errors

• Identify the module: CPU, power supply, I/O, motion, communication.
• Check LED indicators: RUN, ERR, BATT, ALM.
• Open diagnostics in GX Works: Error history provides detailed context.
• Separate PLC logic errors from hardware faults.

CPU and System Errors

Error 4100 - Parameter Error

What it means
Invalid or inconsistent PLC parameters detected during startup or after download.

Common causes

• Incorrect I/O configuration
• Parameter mismatch after module replacement
• Corrupted parameter file

How to fix

• Verify module configuration matches hardware layout
• Rewrite parameters from a known-good project
• Power cycle after confirming parameter integrity

Error 4500 - CPU Module Error

What it means
Internal CPU fault detected. The PLC may stop or enter error mode.

Common causes

• Overheating
• Power instability
• Internal hardware failure

How to fix

• Check cabinet cooling and ventilation
• Verify stable 24VDC and main supply
• If persistent, replace CPU module

Error 6200 - Clock Data Error

What it means
Real-time clock data is invalid or battery-backed memory is compromised.

Common causes

• Battery failure
• Memory corruption

How to fix

• Replace PLC battery
• Reinitialize clock settings
• Restore project if needed

Battery and Memory Errors

Error 6700 - Battery Voltage Low

What it means
Backup battery voltage dropped below safe level.

Common causes

• Aging battery

How to fix

• Replace battery immediately
• Back up program before replacement

Error 2400 - Memory Error

What it means
Internal memory inconsistency detected.

Common causes

• Improper shutdown
• Flash memory degradation

How to fix

• Reload PLC program
• If repeated, replace CPU module

I/O and Module Errors

Error 2100 - I/O Bus Error

What it means
Communication failure between CPU and I/O module.

Common causes

• Loose base connector
• Faulty expansion module
• Incorrect module addressing

How to fix

• Power down and reseat modules
• Check base unit connections
• Replace suspected module if error follows it

Error 2101 - Module Missing

What it means
Configured module not detected by CPU.

Common causes

• Module removed or failed
• Wrong hardware configuration in project

How to fix

• Verify physical module presence
• Match GX Works configuration to actual rack

Communication Errors

Error 2220 - CC-Link Communication Error

What it means
Loss of communication on CC-Link network.

Common causes

• Broken or loose cable
• Incorrect station number
• Noise or termination issues

How to fix

• Inspect and reseat CC-Link cables
• Verify network configuration
• Check termination resistors

Error 4000 - PLC Watchdog Timer Error

What it means
Scan cycle exceeded allowable time. PLC program may be too heavy or stuck in a loop.

Common causes

• Infinite loop in logic
• Excessive communication load

How to fix

• Review recent logic changes
• Optimize scan time
• Test sections of code incrementally

Programming Errors

Error 2050 - Program Syntax Error

What it means
Invalid instruction or syntax detected during compile or execution.

Common causes

• Incorrect ladder instruction
• Unsupported function block

How to fix

• Review compile messages in GX Works
• Correct instruction usage

Error 2060 - Illegal Instruction Execution

What it means
Instruction executed in an invalid context (wrong device type or mode).

Common causes

• Wrong operand type
• Incorrect device range

How to fix

• Verify device types and addressing
• Test instruction in simplified routine

Practical Troubleshooting Strategy

For Mitsubishi PLC systems, always begin with power quality and module seating before diving into code. Many I/O and bus errors are mechanical connection issues. If the error is scan-related (watchdog), focus on recent logic modifications. If battery or memory errors appear, back up immediately and plan preventive replacement.

Consistent maintenance of cabinet cooling, clean power supply, and regular project backups significantly reduces recurring PLC downtime.

About MDCplus

Our key features are real-time machine monitoring for swift issue resolution, power consumption tracking to promote sustainability, computerized maintenance management to reduce downtime, and vibration diagnostics for predictive maintenance. MDCplus's solutions are tailored for diverse industries, including aerospace, automotive, precision machining, and heavy industry. By delivering actionable insights and fostering seamless integration, we empower manufacturers to boost Overall Equipment Effectiveness (OEE), reduce operational costs, and achieve sustainable growth along with future planning.