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Mitsubishi PLC Error Codes
Engineer's Field Guide · Mitsubishi Electric
Mitsubishi PLC Error Codes:
Q-Series & FX-Series Troubleshooting
A flashing ERROR LED on a Mitsubishi CPU tells you something before you even open GX Works. Here's how to read the LED pattern and the numeric error code together, so you fix the actual problem instead of the symptom in front of you.
01 / Overview LED Pattern First
Mitsubishi PLC error codes start with the front-panel LED, not the laptop
Mitsubishi Q-series and FX-series CPUs report faults through a combination of front-panel LED states and numeric error codes readable in GX Works2/3. What makes Mitsubishi's scheme distinct from other platforms is that the LED pattern alone often narrows the problem before a laptop is even connected — genuinely useful when troubleshooting a machine on a noisy floor with no immediate PC access.
On FX3U and FX5U CPUs, a steadily flashing ERROR LED generally indicates a continuable fault — the CPU keeps running in RUN mode, the machine likely keeps producing. A solid-on ERROR LED with the RUN LED off indicates a fatal fault that has already stopped program execution. Confirming which state is showing before connecting a laptop narrows the troubleshooting path significantly: continuable faults are almost never a hardware replacement situation, while fatal faults need faster triage.
This guide breaks down the Mitsubishi PLC error codes IAC's engineers field the most, organized by category, with the LED behavior and first action for each.
02 / Memory Battery & Memory Errors
Battery and memory errors — usually caught before data is actually lost
The 1000-series error codes on Mitsubishi CPUs cover battery and memory backup conditions. These show up frequently on systems that have been running for years without a scheduled battery replacement, and they're often more recoverable than the warning makes them look.
| Error Code | Description | First Action | Common Misdiagnosis |
|---|---|---|---|
| 1010 | Battery error — memory backup battery low or missing | Replace the battery within the buffer window specified for the CPU model; check whether the program and device memory held after replacement | Assumed catastrophic data loss when the program is usually still intact if the battery is replaced before the buffer window expires |
| 1210 | RAM error — internal memory checksum failure | Attempt to reload the program from a backup or memory card first; this can sometimes correct without a hardware swap | Treated identically to a 1010 battery error — but a RAM error is a more serious signal that does sometimes point to genuine hardware failure |
| 1310 | Memory card error — card not detected or read failure | Reseat the memory card; test with a known-good card before assuming the CPU's card slot has failed | Memory card replaced repeatedly when the CPU's card reader, not the card, was the actual point of failure |
The distinction that matters most here: a 1010 battery error is a maintenance event, not a failure — every Mitsubishi CPU is designed to hold memory for a defined window after the battery warning first appears. A 1210 RAM error is a different category and warrants closer inspection before assuming a simple battery swap will resolve it.
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03 / CPU CPU & Watchdog Errors
CPU and watchdog errors — almost always a logic issue, not a slow CPU
The 2000-series codes cover CPU-level operational faults, most commonly watchdog timer events. These are the errors most likely to get a CPU unfairly blamed for being "too slow" or "underpowered," when the actual cause is nearly always in the program.
| Error Code | Description | First Action | Common Misdiagnosis |
|---|---|---|---|
| 2000 | CPU error — WDT (watchdog timer) exceeded | Check scan time against the rated cycle for the CPU model; look for a recently added interrupt routine or heavy loop | CPU replaced for "being too slow" when an interrupt routine was actually looping indefinitely |
| 2002 | Operation circuit error — internal processing fault during instruction execution | Check the specific instruction or function block referenced in the error detail before assuming hardware | Treated as a system-level CPU fault rather than an instruction-level program defect |
| 2200 | I/O module verify error during operation — module configuration changed unexpectedly mid-run | Check for a module that was reseated or replaced without updating the parameter configuration | Assumed CPU instability when a single module's configuration simply didn't match the running parameters |
Why scan time creeps up gradually before a 2000 error appears
A watchdog timeout rarely appears out of nowhere. In most field cases IAC engineers see, scan time has been creeping upward over weeks or months as logic gets added during online edits, until it finally crosses the CPU's rated watchdog threshold. Reviewing scan time history — where available — before assuming a hardware fault often reveals a steady upward trend rather than a sudden spike, which points squarely at the program.
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04 / I/O Module & Parameter Errors
Module and parameter errors — configuration mismatches, not dead hardware
The 7000-series self-diagnosis errors cover module verification and parameter mismatches. These are some of the most common errors after any kind of rack reconfiguration, and they are frequently misread as a failed module when the module itself is functioning correctly.
| Error Code | Description | First Action | Common Misdiagnosis |
|---|---|---|---|
| 7000 | I/O module verify error — installed module does not match configured parameters | Compare loaded parameters against the project file; confirm physical rack layout matches the configuration exactly, slot by slot | Module physically swapped before checking whether parameters simply didn't match after a rack reconfiguration |
| 7001 | Module installation position error — module installed in an unexpected slot | Check the slot assignment against the configured I/O map; a module installed one slot off from its configured position triggers this | Module assumed defective when it was simply seated in the wrong physical slot during a recent rack rebuild |
| 7002 | Intelligent function module parameter error — parameter values out of valid range | Check parameter settings for the specific intelligent module (e.g., analog, positioning) against its valid operating range | Module replaced when a single out-of-range parameter — often introduced during a project edit — was the actual cause |
| 7003 | Base unit/extension cable error — extension base not detected | Check the extension cable connection and base unit power before assuming a CPU-side fault | CPU questioned for an extension rack issue, when the extension base's own power supply or cable was the problem |
The throughline across this category: 7000-series errors describe a mismatch, not a failure. Before ordering a replacement I/O module, it's worth the extra two minutes to verify the parameter configuration in GX Works actually matches what's physically installed — especially after any maintenance event that touched the rack.
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05 / Networking Link & Network Errors
Link and network errors — check termination and station addressing first
The 4000-series codes cover link and network communication failures across CC-Link, Ethernet, and serial connections. These are far too often blamed on the CPU's onboard communication port when the actual cause is a wiring or addressing issue on the network itself.
| Error Code | Description | First Action | Common Misdiagnosis |
|---|---|---|---|
| 4000 | CC-Link communication error — station not responding on the link | Check termination resistors at both ends of the CC-Link segment and confirm no duplicate station numbers exist | CPU's communication processor blamed when a missing termination resistor was the actual cause |
| 4002 | Ethernet communication error — target device unreachable | Check cable continuity, switch port status, and IP address configuration before assuming a CPU port failure | Onboard Ethernet port assumed failed when a switch reboot had simply reset a DHCP-assigned address outside the expected range |
| 4004 | Serial communication error — RS-422/485 link timeout | Check wiring polarity, baud rate match between devices, and termination on the serial segment | Serial communication module replaced when a baud rate mismatch after a device firmware update was the actual cause |
| 4012 | Duplicate station number detected on CC-Link or Ethernet network | Check for a recently added device; station numbers and IP addresses must be unique across the network segment | Existing, correctly configured device suspected of failure when a newly added device had a conflicting address |
Before blaming the CPU's communication port, check these three things
- Termination resistors present and correctly placed at both physical ends of the CC-Link or serial segment
- No duplicate station numbers or IP addresses anywhere on the network — especially after any recent device addition
- Cable continuity and connector seating at every junction between the CPU and the device reporting the fault
06 / Decision Point When It's a Genuine CPU Fault
When the error code actually does mean "replace the CPU"
Most Mitsubishi error codes point to a configuration mismatch, a network issue, or a program defect. But some genuinely indicate it's time to source a replacement CPU.
| Signal | Why It Points to Hardware |
|---|---|
| 1210 RAM error recurs after reloading the program from a known-good backup | If the checksum failure persists with a confirmed-good program reload, the internal memory hardware itself is suspect |
| 1310 memory card error persists across multiple known-good cards | If a card that reads correctly in another CPU still fails in this one, the card reader hardware has failed, not the card |
| Solid-on ERROR LED with RUN LED off and no correlating program or configuration change | This LED pattern indicates a fatal fault that stopped execution outright — without a logic or config trigger, hardware is the likely cause |
| CPU fails to complete its power-on self-test consistently across multiple power cycles | A repeatable self-test failure before the program even loads is a hardware-layer issue, not software |
| Fault persists after isolating and ruling out every module, network, and program-level cause | Process of elimination — once every peripheral and logic cause is excluded, the CPU is the remaining variable |
For an older or discontinued Mitsubishi CPU — a legacy Q02H or an early-generation FX3U — confirming the fault is genuinely hardware before ordering matters, since replacement units for end-of-life parts may be refurbished or new-surplus rather than current production. IAC verifies part number and firmware compatibility on every order before it ships.
Confirmed a genuine CPU fault? Submit your part number — IAC verifies compatibility before shipping
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07 / Sourcing Getting the Replacement Fast
Diagnosed the fault. Now get the right Mitsubishi part, shipped today.
IAC stocks Mitsubishi Q-series and FX-series CPUs, I/O modules, intelligent function modules, and GOT HMIs — the exact hardware behind most of the error codes covered above. Whether the fault traces to a battery warning, a parameter mismatch after a rack rebuild, a network addressing conflict, or a genuine internal CPU failure, the right replacement is sourced and verified before it ships.
Warranty and verification
Every Mitsubishi unit IAC ships carries a 2-year in-service warranty — twice the industry standard for refurbished industrial components. CPUs are tested with a project load; I/O modules are verified for channel-level functionality before they leave the warehouse. If an error code reading is ambiguous, IAC's engineers can help confirm whether a hardware replacement is actually warranted.
Same-day shipping
In-stock Mitsubishi parts ordered before 4:00 PM Eastern ship same day. For urgent line-down situations, call (877) 727-8757 during business hours — quote turnaround is typically under five minutes. You can also submit a part number via the quote form ↗ or email sales@iac.us.com.
Mitsubishi Q-series · FX-series · GOT HMIs — all in stock, 2-year warranty
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