Troubleshoot your Mitsubishi MR-J3 servo amplifier with our updated fault code guide. Learn to interpret AL. codes, use MR Configurator2, and resolve battery/encoder errors.

MR-J3 Fault Codes Reference Table

Below is the complete list of servo alarms and warnings. Use this table to match the flashing LED code on the amplifier faceplate with the internal hardware or parameter issue.

Mitsubishi MELSERVO MR-J3 Fault Codes List

Fault Code and Meaning	Cause and Remedy
AL.10 Undervoltage	Cause: 1. Power supply voltage is low. 2. Shortage of power supply capacity caused voltage drop at start. 3. Bus voltage dropped to 200VDC/158VDC/380VDC or less. 4. Instantaneous control power failure of 60ms or longer. 5. Faulty parts in the servo amplifier. 6. Waveform of power supply voltage is distorted. Remedy: Check the power supply voltage (MR-J3-A: 160VAC/200VDC min). Change the servo amplifier (if faulty parts). Set parameter No.PC27 to "0001" (if waveform is distorted/high impedance).
AL.12 Memory error 1 (RAM)	Cause: Faulty parts in the servo amplifier (RAM, memory fault). Remedy: Change the servo amplifier.
AL.13 Clock error	Cause: Printed board fault. Remedy: Change the servo amplifier.
AL.15 Memory error 2 (EEP-ROM)	Cause: 1. Faulty parts in the servo amplifier. 2. The number of write times to EEP-ROM exceeded 100,000. Remedy: Change the servo amplifier.
AL.16 Encoder error 1 (At power on)	Cause: 1. Encoder connector (CN2) disconnected. 2. Encoder cable type (2-wire/4-wire) selection incorrect in parameters. 3. Encoder cable faulty (broken or shorted). 4. Encoder fault. 5. A servo motor other than MR-J3 series is connected. 6. Communication error due to external noise. Remedy: Connect connector correctly. Correct the setting in parameter No. PC22. Repair or change the encoder cable. Change the servo motor. Check the combination of servo amplifier and motor. Ground correctly or take noise reduction measures.
AL.17 Board error	Cause: CPU/parts fault in the servo amplifier. Remedy: Change the servo amplifier.
AL.19 Memory error 3 (Flash ROM)	Cause: ROM memory fault in the servo amplifier. Remedy: Change the servo amplifier.
AL.1A Motor combination error	Cause: Incorrect combination of servo amplifier and servo motor. Remedy: Check the combination of the servo amplifier and the servo motor.
AL.20 Encoder error 2 (during runtime)	Cause: 1. Encoder cable disconnected. 2. Encoder cable fault (broken or shorted). 3. Encoder detected high acceleration rate due to oscillation. 4. Encoder fault. 5. Communication error due to external noise. Remedy: Connect the servo motor encoder connector (CN2) correctly. Repair or change the cable. Decrease the position loop gain or reduce auto tuning response. Change the servo motor. Ground correctly or take noise reduction measures.
AL.21 Encoder error 3 (during runtime)	Cause: Detection circuit error in encoder. Remedy: Change the servo motor.
AL.24 Main circuit error	Cause: 1. Power input wires and servo motor power wires are in contact. 2. Short or ground fault in servo motor power cable. 3. Servo amplifier fault. 4. Servo motor fault. 5. External dynamic brake fault. 6. External noise caused erroneous overcurrent detection. Remedy: Modify the wiring. Repair the cable. Change the servo amplifier. Change the servo motor. Check dynamic brake interlock parameters or replace external brake. Ground correctly or take noise reduction measures.
AL.25 Absolute position erase	Cause: 1. Voltage drop in encoder (Battery disconnected). 2. Battery voltage low. 3. Loose battery connection or battery fault. 4. Encoder cable fault. 5. Encoder fault. 6. Home position not set. Remedy: Power cycle and perform home position setting again. Change the battery. Repair or change the encoder cable. Change the servo motor.
AL.30 Regenerative error	Cause: 1. Incorrect setting of parameter No. PA02. 2. High-duty operation or continuous regenerative operation. 3. Bus voltage is abnormal (High). 4. Built-in regenerative resistor/option not connected. 5. Built-in regenerative resistor/option faulty. 6. Regenerative transistor fault (Servo amp fault). 7. Regenerative circuit fault (Servo amp fault). Remedy: Set PA02 correctly. Reduce positioning frequency, reduce load, or use larger capacity option. Check the power supply. Connect correctly. Change the servo amplifier or regenerative option.
AL.31 Overspeed	Cause: 1. Input command pulse frequency is too high. 2. Small acceleration/deceleration time constant. 3. Servo system is unstable causing overshoot. 4. Electronic gear ratio is large (PA06, PA07). 5. Encoder faulty. Remedy: Set command pulse frequency correctly. Increase acceleration/deceleration time constant. Re-set servo gain or load inertia moment ratio. Set gear ratio correctly. Change the servo motor.
AL.32 Overcurrent	Cause: 1. Short or ground fault at servo motor power cable. 2. External dynamic brake fault. 3. Servo amplifier fault. 4. Servo motor fault. 5. External noise caused erroneous operation. 6. Encoder fault. Remedy: Repair the cable. Replace the external dynamic brake. Change the servo amplifier. Change the servo motor. Ground correctly or take noise reduction measures.
AL.33 Overvoltage	Cause: 1. Regenerative option not used. 2. PA02 setting is "not used" despite option attached. 3. Lead of built-in resistor or option is open/disconnected. 4. Wire breakage of regenerative resistor/option. 5. Capacity of regenerative option insufficient. 6. Jumper across BUE-SD removed. 7. High impedance at main circuit power supply. 8. Ground fault in servo motor power. 9. Power supply voltage high. 10. Servo amplifier fault. Remedy: Use the regenerative option/Set parameters correctly. Connect leads/jumpers correctly. Change the servo amplifier or regenerative option. Add regenerative option capacity. Correct the wiring. Check the power supply.
AL.35 Command pulse frequency error	Cause: 1. Input command pulse frequency is too high. 2. Noise entered command pulses. 3. Command device failure. Remedy: Change the command pulse frequency to a lower value. Take action against noise. Change the command device.
AL.37 Parameter error	Cause: 1. Regenerative option not used but selected in PA02. 2. Incorrect setting for PC22 on high drive units. 3. EEP-ROM write limit exceeded (100,000 times). 4. Servo amplifier fault caused parameter rewrite. Remedy: Set parameter No. PA02 correctly. Set parameter No. PC22 correctly and cycle power. Change the servo amplifier.
AL.45 Main circuit device overheat	Cause: 1. Ambient temperature of servo amplifier is over 55°C (131°F). 2. Used beyond the specifications of close mounting. 3. The power supply was turned on and off continuously by overloaded status. 4. Foreign matter caught in a cooling fan or heat sinks. 5. Servo amplifier fault (occurs immediately after power-on). Remedy: Check environment so that ambient temperature is 0 to 55°C (32 to 131°F). Use within the range of specifications. Review the drive method. Clean the cooling fan or the heat sinks. Change the servo amplifier.
AL.46 Servo motor overheat	Cause: 1. Ambient temperature of servo motor is over 40°C (104°F). 2. Servo motor is overloaded. 3. Thermal sensor in encoder is faulty. Remedy: Check environment so that ambient temperature is 0 to 40°C (32 to 104°F). Reduce load, check operation pattern, or use servo motor that provides larger output. Change the servo motor.
AL.47 Cooling fan alarm	Cause: 1. Cooling fan life expiration. 2. Foreign matter caught in the cooling fan stopped rotation. 3. The power supply of the cooling fan failed. Remedy: Change the cooling fan of the servo amplifier. Remove the foreign matter. Change the servo amplifier.
AL.50 Overload 1	Cause: 1. Servo amplifier is used in excess of its continuous output current. 2. After Overload 2 (AL.51) occurred, power was cycled and overload operation repeated. 3. The servo system is unstable and causes oscillation or hunting. 4. Encoder fault. Remedy: Reduce load, check operation pattern, check brake, check machine friction, or use larger output motor/amplifier. Reduce load, check operation pattern, use larger servo motor. Repeat auto tuning, change response setting, or manual gain adjustment. Check coupling. Change the servo motor.
AL.51 Overload 2	Cause: 1. Servo amplifier fault. 2. The servo system is unstable and causes oscillation or hunting. 3. Machine struck something. 4. Incorrect connection of servo motor (U, V, W mismatch). 5. Encoder fault. 6. A power cable is disconnected. 7. Servo motor fault. Remedy: Change the servo amplifier. Repeat auto tuning, change response setting, or manual gain adjustment. Check coupling. Check operation pattern, install limit switches, check electromagnetic brake. Connect terminals U, V, W correctly. Change the servo motor. Repair the cable. Change the servo motor.
AL.52 Error excessive	Cause: 1. Acceleration/deceleration time constant is too small. 2. Torque limit values (PA11/PA12) are too small. 3. Torque shortage due to power supply voltage drop. 4. Position loop gain 1 (PB08) value is small. 5. Servo motor shaft was rotated by external force. 6. Machine struck something. 7. Encoder fault. 8. Incorrect connection of servo motor. 9. A power cable is broken. 10. Command input when torque limit is "0". Remedy: Increase the acceleration/deceleration time constant. Increase the torque limit value. Check power supply capacity or use larger motor. Increase set value and adjust to ensure proper operation. Increase torque limit, reduce load, or use larger motor. Check operation pattern and install limit switches. Change the servo motor. Connect correctly. Repair the cable. Set the torque limit to the proper value.
AL.8A Serial communication time-out error	Cause: 1. Communication cable breakage. 2. Communication cycle longer than regulated time. 3. Incorrect protocol. Remedy: Repair or change the communication cable. Shorten the communication cycle. Correct protocol.
AL.8E Serial communication error	Cause: 1. Communication cable fault (Open cable or short circuit). 2. Communication device (e.g. personal computer) faulty. 3. A character code is faulty. 4. A command is faulty. 5. A data No. is faulty. Remedy: Repair or change the cable. Change the communication device. Check the character codes. Check the commands. Check the data No.
88888 Watchdog (CPU, parts faulty)	Cause: 1. Fault of parts in servo amplifier (occurs if power on after cable disconnection). 2. The CPU in the servo motor malfunctioned due to external noise. Remedy: Change the servo amplifier. Check that the amplifier is not influenced by noise (valves, contactors, relays). Check the grounding of the servo amplifier and motor.
AL.92 Battery cable disconnection warning	Cause: 1. Battery cable is open. 2. Battery voltage supplied to encoder fell to about 3V or less. 3. An encoder cable is broken. Remedy: Repair cable or change it. Change the battery. Repair or replace the encoder cable.
AL.96 Home position setting warning	Cause: 1. The position is out of in-position range at the home position setting. 2. A command pulse is input during the home position setting. 3. Creep speed high. Remedy: Set the home position within the in-position range. Input the command pulse after the home position setting. Reduce creep speed.
AL.99 Stroke limit warning	Cause: The forward rotation stroke end (LSP) or reverse rotation stroke end (LSN) is turned off at the command. Remedy: Review the moving range to avoid turning off LSP/LSN.
AL.9F Battery warning	Cause: Battery voltage fell to 3.2V or less. Remedy: Change the battery.
AL.E0 Excessive regenerative warning	Cause: Regenerative power increased to 85% or more of permissible power of built-in resistor or option. Remedy: Reduce frequency of positioning. Change the regenerative option for one with larger capacity. Reduce load. Replace servo amplifier/motor with larger capacity.
AL.E1 Overload warning 1	Cause: Load increased to 85% or more of overload alarm 1 or 2 occurrence level. Remedy: Refer to remedies for AL.50 and AL.51.
AL.E3 Absolute position counter warning	Cause: 1. Noise entered the encoder. 2. Encoder faulty. 3. Travel distance from home position exceeded maximum revolution range. Remedy: Take noise suppression measures. Change the servo motor. Make home position setting again.
AL.E5 ABS time-out warning	Cause: 1. Programmable controller ladder program incorrect. 2. Reverse rotation start (ST2) / Limiting torque (TLC) improper wiring. Remedy: Contact the program. Connect properly.
AL.E6 Servo emergency stop warning	Cause: External emergency stop (EMG) was made valid (turned off). Remedy: Ensure safety and deactivate emergency stop.
AL.E8 Cooling fan speed reduction warning	Cause: 1. Cooling fan life expiration. 2. The power supply of the cooling fan is broken. 3. Foreign matter is caught in the cooling fan. Remedy: Change the cooling fan. Change the servo amplifier. Remove the foreign matter.
AL.E9 Main circuit off warning	Cause: Servo-on (SON) was switched on with main circuit power off. Remedy: Switch on main circuit power.
AL.EA ABS servo-on warning	Cause: 1. Programmable controller ladder program incorrect. 2. Servo-on (SON) improper wiring. Remedy: Correct the program. Connect properly.
AL.EC Overload warning 2	Cause: Current exceeding the rating flew intensively in U, V, W phases repeatedly. Remedy: Reduce the positioning frequency. Reduce the load. Replace the servo amplifier/motor with larger capacity.
AL.ED Output watt excess warning	Cause: Continuous operation performed with output wattage exceeding 150% of rated output. Remedy: Reduce the servo motor speed. Reduce the load. Replace the servo amplifier/motor with larger capacity.

 

How to Read MR-J3 Faults via MR Configurator2

While the LED display gives you the basic code (e.g., AL 32), connecting your PC to the amplifier using the MR Configurator2 software provides deep diagnostic data essential for 2026 maintenance standards.

• Alarm Display: The software shows the current alarm details, including the specific axis and sub-codes that the LED display cannot show.
• Alarm History: The MR-J3 stores the last 6 alarms with timestamps (operating time). This is crucial for identifying patterns in intermittent failures.
• Drive Recorder: This is a powerful feature that captures waveform data (speed, torque, current) from before and after the alarm occurred. It allows you to visualize exactly what the motor was doing the moment it tripped.

General Troubleshooting Steps

Servo systems are sensitive. Before resetting an alarm, verify the following:

1. Check the Battery: If you see AL.25 (Absolute Position Lost) or 9F (Battery Warning), the absolute position battery is dead or disconnected. Do not power off until you have a replacement plan, or home position will be lost.
2. Inspect Encoder Cables: For communication errors (AL.16, AL.20), check the encoder cable for breaks, especially if the motor is on a moving gantry (flex cable fatigue).
3. Mechanical Checks: For Overload (AL.50, AL.51), ensure the machine axis is not mechanically jammed and the brake is releasing correctly.

Frequently Asked Questions (FAQ)

Q: What is the difference between a Servo Warning and a Servo Alarm?

A: A Warning (codes usually indicated by alphanumeric flashes like ‘9F’ or ‘E9’) indicates a potential issue—such as low battery or slight overload—but the servo continues to operate. An Alarm (codes starting with ‘AL.’, such as AL.32) stops the servo motor immediately (Dynamic Brake or Coast) to prevent damage.

Q: How do I reset a Mitsubishi MR-J3 Alarm?

A:

• Power Cycle: Switch the control circuit power (L11, L21) OFF and then ON. This is required for major faults.
• Current Alarm Reset: For minor alarms, press the “SET” button on the front if in test mode, or toggle the RES (Reset) digital input signal.
• Software Reset: Click the “Alarm Reset” button within MR Configurator2.

Q: Why does my drive show “Ab” or “b0”?

A: These are not faults. They are status indicators. “Ab” typically indicates the drive is in a ready state waiting for a servo-on signal, while specific codes during startup indicate the boot sequence or communication status with the controller.