MELSEC iQ-R Fault Diagnostics & Recovery Guide (R04-R120)

The MELSEC iQ-R series gives you more diagnostic data than any previous Mitsubishi platform — module-level error logs, an SD card event recorder, and GX Works3 system monitor running in parallel. The downside is that operators looking at a flashing ERROR LED rarely know which of those three sources to read first, so an iQ-R rack often sits faulted while a maintenance team chases the wrong root cause.

This guide covers the diagnostic workflow that gets an R04, R08, R16, R32, or R120 CPU back into RUN as quickly as possible: reading the ERROR LED pattern, pulling the right error code, interpreting the SD card log, and deciding when to reset, reinitialize, or swap a module.

ERROR code quick-reference

iQ-R error codes are 4-digit hex values grouped by source. The first digit identifies the subsystem; the remaining three identify the specific fault. Knowing the group narrows the search before you even open GX Works3.

Three LEDs on the CPU front panel tell you the severity before you connect a laptop:

• ERROR off — no detected fault.
• ERROR flashing — continuation error. The CPU is still scanning. You have time to read the log without losing process state.
• ERROR steady on — stop error. The CPU has halted. Outputs are in their configured failsafe state.
• P RUN flashing with ERROR off — reset in progress or initial start-up. Wait it out before assuming a fault.

Reading error codes with GX Works3

GX Works3 is the canonical tool for iQ-R diagnostics. The workflow:

1. Connect over USB or Ethernet. The default IP for an unconfigured R-series CPU is 192.168.3.39.
2. Online → Diagnostics → Module Diagnostics. The system tree shows a red icon next to any module reporting an error.
3. Click the faulted module. The Error History tab lists each event with timestamp, error code, and detailed cause text.
4. For CPU-level events, open CPU Module Diagnostics from the same menu. This pulls the most recent 16 errors plus the continuation/stop classification.
5. Use Event History for a longer view (up to 15,000 events) — this is the SD card log surfaced in GX Works3.

Without GX Works3 access, you can read the most recent error code from the 7-segment display on R04EN/R08EN/R16EN/R32EN/R120EN Ethernet-built-in CPUs by pressing the MODE button until the display cycles to ERR. Older R-series CPUs without the integrated display require the engineering tool.

CPU errors (1000-2FFF)

1010 — Memory diagnostic error

RAM check failed at power-up or watchdog reset. Most often a transient caused by a marginal power feed; if it repeats after a clean power cycle, the CPU memory is failing. Confirm 24 VDC at the base unit is within 19.2-30 V and stable. If the error reappears within a week, replace the CPU.

1080 — Battery error

Q6BAT or Q7BAT battery voltage low. Continuation error — the CPU keeps running, but program memory backup and clock data are at risk during power loss. Replace the battery within one week. iQ-R supports hot-swapping the battery while the CPU is powered.

2400 — I/O assignment mismatch

The actual module installed in a slot does not match the I/O configuration in the parameter file. Common after a module swap or a wrong-slot insertion. Open Parameter → System Parameter → I/O Assignment, click Read PLC Data to pull the actual configuration, and compare. Either correct the physical module or update the parameters and write to PLC.

2450 — Module configuration error

An intelligent function module (analog, positioning, network) has parameters that conflict with hardware capability — such as setting an RD75D4 to 8 axes when only 4 are physical. Open the module parameter dialog and reset to defaults, then reconfigure.

Network errors (7000-7FFF)

7400 — CC-Link IE TSN station number duplicate

Two stations on the same CC-Link IE TSN network are configured with the same station number. The second station to come online flags 7400 and stays offline. Open each remote module's parameter and verify station numbers are unique. Power-cycle the offending module after correction.

7500 — Ethernet IP address conflict

Another device on the subnet is using the CPU's IP. Common after replacing a CPU without re-imaging its IP. Use Find/Replace IP Address in GX Works3 or change the conflicting device. The CPU does not auto-recover; it stays at 7500 until corrected and the network module is reset.

7600 — CC-Link IE field cyclic data missing

A remote slave stopped responding. Could be cable damage, a powered-off remote, or PoE budget exceeded. Check the link LED on each segment. Cyclic data resumes automatically when the slave reconnects unless you have configured the master to require a manual restart.

I/O and intelligent module errors (2400-25FF)

2420 — I/O module access error

CPU cannot access the module at the configured slot. Causes: module physically removed during run, base unit connector dirty, or backplane current overload. Power down, reseat the module, and check the base unit power supply (R61P, R62P, R63P, R64P, R64ENCPU) is sized for the rack. The R64P at 6.5 A handles most heavily-loaded 12-slot bases; lighter racks use R63P (4.1 A) or R62P (3.5 A).

2520 — Module hardware error

The module reports an internal failure (ADC reference drift, output FET failure, comm ASIC error). Replace the module. Before doing so, swap it with a known-good identical module in another slot to confirm the fault travels with the part — this rules out a base-unit problem.

Program errors (3000-4FFF)

3010 — Program parameter mismatch

The program written to the CPU references devices or labels that do not exist in the current parameter set. Usually after a partial download. Re-download the full project (Online → Write to PLC, select all).

4100 — Operation error

An instruction encountered an illegal operand: division by zero, BCD operation on non-BCD data, array out of bounds. The error history records the program name and step number. Open the program at that step and add bounds checking. 4100 is a continuation error by default but can be configured to stop the CPU in System Parameter → CPU Parameter → Operation Error.

4220 — Watchdog timer over

Scan time exceeded the configured watchdog (default 200 ms). Caused by a long FOR/NEXT loop, blocking communication instruction, or recursive function block. Open Online → Monitor → Scan Time to find the offending program. If scan time is genuinely needed, raise the watchdog in System Parameter; otherwise refactor the long-running logic.

SD card event log

The SD card slot on every iQ-R CPU records an event log when configured. Use it to recover diagnostic data after a power loss or to investigate intermittent faults that escape the 16-entry on-CPU history.

• Format the SD card from GX Works3 (Online → User Data Operation → Format SD) — do not format from a PC.
• Enable event logging in CPU Parameter → SD Memory Card Setting. Choose CSV or binary format. CSV is human-readable; binary is faster for high-frequency events.
• Pull the SD card with the CPU powered off, or use the SD memory card lock function (X1 special relay) to suspend writes before removal during runtime.
• On a PC, open the .CSV file directly. Each row is a timestamp, error code, module slot, and detail string. Filter on the time window of interest.

The log holds tens of thousands of events on a typical 4 GB card. Keep an SD card in every iQ-R CPU even if you never plan to use it for recipes — the diagnostic value alone justifies it.

CPU reset and initialization

Three recovery actions are available, in order of disruption:

Reset (RESET position on the RUN/STOP switch, hold 1 second)

Clears the current error state and restarts the scan. Retains program, parameters, and latched device values. Use after correcting a continuation error (4100, 7600) or clearing a transient. Does not fix configuration mismatches.

Memory clear (Online → CPU Memory Operation → Clear Memory)

Erases the program, parameters, and device memory in the CPU. Used after a planned migration or when the CPU has unknown legacy logic that conflicts with a new project. The CPU returns to its factory state with default IP. Always have a verified project backup before a memory clear.

Initialization (CPU Parameter → Initialization Setting)

Restores parameters to factory defaults but preserves the program. Use sparingly — most error states do not require initialization, and the wrong reset wipes carefully tuned communication settings.

Module replacement decision tree

Use this sequence when GX Works3 fingerprints a hardware fault:

• 2520, 2530, recurring 2420 on one slot — module is failing. Order replacement by exact part number from the side label, including the suffix (R32CPU vs R32ENCPU is not interchangeable).
• 1010, 1080 with no other context — CPU. Check battery first (1080), then full CPU swap (1010 recurring).
• Multiple modules in one base showing 2420 simultaneously — base unit (R36B, R310B, R312B) or its power supply. Inspect the connector finger contacts and confirm power supply current rating. Replace the base only if reseating fails.
• Network errors confined to one segment — cabling, switch, or media converter, not the CPU module. Replace the segment hardware before condemning the network module.

For replacement parts, verify the firmware revision on the new module is equal to or newer than the failed unit. iQ-R modules running mismatched firmware in CPU redundancy pairs raise a 1810 mismatch error at startup. Browse our Mitsubishi MELSEC inventory for available CPUs, base units, power supplies, and I/O cards, or check controllers for cross-platform replacement options.

Preventive practices

• Annual battery replacement on every iQ-R CPU regardless of error state. Q6BAT lifetime in 25°C ambient is roughly 5 years; hot panels see 3.
• Export and archive the parameter set after every commissioning change. Without it, a memory clear is a project rebuild.
• Confirm SD logging is enabled at handover. Many sites discover the slot is empty only after they need the log.
• Trend the scan time through GX LogViewer or to the CPU's own event log. A scan time creeping toward the watchdog over months is the earliest indicator of bloated logic or a struggling network module.
• Stock one spare CPU per running model. R04 cannot substitute for R32 in a critical line — capacity differences mean the program will not download.

FAQ

How do I clear an error without losing my program?

For continuation errors, correct the root cause (network cable, parameter, division operand) and the error self-clears on the next scan or after a switch reset. For stop errors, fix the cause first, then move the RUN/STOP switch to RESET for one second and back to RUN. The program and device values are retained. A memory clear is never required to acknowledge a normal error.

Can I read the error code without GX Works3?

The Ethernet-built-in CPUs (R04EN, R08EN, R16EN, R32EN, R120EN) display the most recent error code on the 7-segment front panel — press MODE to cycle to ERR. Plain R04, R08, R16, R32, and R120 CPUs require GX Works3 or a HMI mapped to the special relay/special register error area (SD0, SD1).

Will an R32CPU run a project written for an R16CPU?

Yes — iQ-R CPUs are upward-compatible within the standard family. A project sized for R16 (1600 KB program capacity) downloads and runs on R32 (2000 KB) or R120 (40 MB) without changes. The reverse is not true: a project that exceeds the smaller CPU's program or device capacity will fail the download with a 3050 error.

What does a flashing P RUN with ERROR off mean?

The CPU is in a transitional state — initializing after power-up, downloading a project, or in the middle of a reset. Wait 30 seconds. If P RUN does not go solid, check the CPU operating mode switch and the project download status in GX Works3. A persistent flash usually means the parameter or program write was interrupted.

How long does the iQ-R CPU keep error history without an SD card?

The CPU retains the last 16 error events in internal memory, backed up by the battery. Without the SD card you lose visibility into anything older than those 16 events, and you lose all events that occur if the battery dies during a power outage. Adding even a 2 GB industrial SD card extends practical history to months or years.

Need a replacement R-series CPU, base unit, or I/O module? Browse our Mitsubishi MELSEC catalog for in-stock iQ-R CPUs, R6/R3/R4-series power supplies, and intelligent function modules, or check controllers and drives and motion control for adjacent products. Send the part number from the module side label for a same-day quote.