Troubleshoot your Mitsubishi MD-AX520 servo drive unit with our updated fault code guide. Learn to interpret E.OC, E.OV, and AL. codes, view alarm history, and reset trips.

MD-AX520 Fault Codes Reference Table

Below is the complete list of protective functions and alarm codes. Use this table to correlate the alphanumeric display on your drive with the specific hardware or parameter issue.

Mitsubishi MD-AX520 Fault Codes List

Fault Code and Meaning	Cause and Remedy
E.OC1 Acceleration time overcurrent	Cause: The acceleration torque is beyond the drive unit capability; Outputs U, V, and W are in a short circuit or ground fault; The motor restarted during coasting; Or start signal given with only R1/S1 power. Remedy: Increase the acceleration time. Change the acceleration/deceleration pattern to an S shape. Check the motor winding resistance. Check the connection cables for damage. Restart the motor after a complete stop. Supply main circuit power (R, S, T).
E.OC2 Constant speed time overcurrent	Cause: Excessive load was applied instantaneously; Outputs U, V, and W resulted in a short circuit or ground fault during constant-speed operation; Drive capacity does not match motor. Remedy: Check the motor winding resistance. Check the connection cables for damage. Ensure drive unit capacity matches the motor.
E.OC3 Deceleration time overcurrent	Cause: The deceleration torque is beyond the drive unit capability; Outputs U, V, and W resulted in a short circuit or ground fault during deceleration; Mechanical brake operates too early. Remedy: Increase the deceleration time. Change the acceleration/deceleration pattern to an S shape. Check the motor winding resistance. Check the connection cables for damage. Delay the brake operation timing.
E.GF Output side ground fault overcurrent	Cause: Any of outputs U, V, W is in a ground fault. Remedy: Check for ground faults in the motor and wiring.
E.OV1 Acceleration time overvoltage	Cause: Surge compounded with power during acceleration; Regenerative energy handling capability is insufficient. Remedy: Install a reactor. Fit a surge suppressor to the surge source. Fit the regenerative brake option.
E.OV2 Constant speed time overvoltage	Cause: Surge compounded with power; Load increased suddenly activating stall prevention; Regenerative energy capability insufficient. Remedy: Install a reactor. Fit a surge suppressor to the surge source. Fit the regenerative brake option.
E.OV3 Deceleration time overvoltage	Cause: Deceleration torque beyond drive capability; Regenerative energy capability insufficient; Surge compounded with power during deceleration. Remedy: Increase the deceleration time. Change acceleration/deceleration pattern to S shape. Fit the regenerative brake option. Install a reactor or surge suppressor.
E.THT Electronic overcurrent protection (Transistor)	Cause: The motor and drive unit are operated under overload. Remedy: Reduce the load. Increase the capacities of the motor and drive unit.
E.THM Electronic overcurrent protection (Thermal)	Cause: The motor and drive unit are operated under overload. Remedy: Reduce the load. Increase the capacities of the motor and drive unit.
E.IPF Instantaneous power failure	Cause: An instantaneous power failure occurred (15 to 100ms). Remedy: Check power supply stability. Reset the alarm.
E.UVT Undervoltage	Cause: Large-capacity equipment started nearby; Jumper across terminals P/+-P1 disconnected. Remedy: Connect the jumper or DC reactor. Check power supply capacity.
E.FIN Fin overheat	Cause: Heat sink temperature exceeded permissible value; Cooling fan failed. Remedy: Check ambient temperature. Replace cooling fan if failed. Clean heat sink fins.
E.OHT External fault	Cause: The external relay or equivalent component connected to the terminal assigned to OH signal operated. Remedy: Check external device/relay triggering the fault. Check wiring of the OH signal terminal.
E.PUE PU disconnection	Cause: Connection fault of operation panel/parameter unit; RS-485 communication interrupted; Comm retries exceeded. Remedy: Check connection of the operation panel. Check RS-485 wiring and parameters (Pr. 75, Pr. 121).
E.RET Retry excess	Cause: The number of alarms that occurred exceeded the retry setting. Remedy: Investigate the root cause of the recurring alarms. Check Retry count parameter (Pr. 67).
E.OLT Stall stop	Cause: Motor overload activated stall prevention consecutively; E.OLT output selected in Pr. 156. Remedy: Reduce the load. Increase capacities of motor and drive unit. Check Pr. 156 setting.
E.LF Phase failure	Cause: A phase failure occurred in any of the outputs U, V, W. Remedy: Check wiring for breaks or loose connections. Check motor windings.
E.P24 P24 Short Circuit	Cause: The PC terminal output of the control circuit is in a short circuit. Remedy: Check the wiring connected to the PC terminal. Reset via method other than RES terminal.
E.CTE Panel power supply	Cause: The power supply cable of the operation panel or parameter unit is in a short circuit. Remedy: Check the cable wiring for shorts.
E.OPT Option fault	Cause: The plug-in option fitted is other than specified; Power switched on with high power factor converter setting (Pr. 30 = 2). Remedy: Check the installed option card. Check Parameter 30 setting.
E.CPU / E.6 / E.7 CPU fault	Cause: The CPU malfunctioned or failed. Remedy: If the same message reappears after reset, replace the drive unit.
E.PE Storage device fault	Cause: The storage device failed. Remedy: If the same message reappears after reset, replace the drive unit.
E.BE Brake circuit fault	Cause: The regenerative brake circuit failed. Remedy: Immediately switch power off if E.BE reappears. Leaving power on will overheat the brake resistor. Replace drive unit if fault persists.
OL Overload 1 (Alarm)	Cause: Acceleration/Constant speed/Deceleration time overcurrent; Stall stop activated. Remedy: Reduce load. Check Pr. 22 setting.
oL Overload 2 (Alarm)	Cause: Main circuit DC voltage more than 390V (Deceleration-time overvoltage). Remedy: Increase deceleration time. Check regenerative brake capability.
RB Brake duty (Alarm)	Cause: More than 85% of the permissible value (Deceleration-time overvoltage). Remedy: Reduce braking duty cycle. Check brake resistor sizing.
TH Motor overheat (Alarm)	Cause: More than 85% of the electronic overcurrent protection level. Remedy: Reduce motor load. Check cooling.
FN Fan failure (Alarm)	Cause: Cooling fan fault; Fin overheat. Remedy: Replace cooling fan. Clean heatsink.
Err Operation error	Cause: Parameter changing during external operation; Value outside range; RES signal remains ON; Connection fault. Remedy: Perform parameter changes in PU mode. Check valid parameter ranges. Turn off RES signal. Check panel fitting status.
PS Emergency stop operation	Cause: STOP/RESET key was pressed during external operation. Remedy: Reset cannot be made by normal method. Refer to Pr. 75 for reset procedures.
E.rE4 Copy error	Cause: Parameters were copied to a different model. Remedy: Ensure source and destination drive models match.

 

How to Read MD-AX520 Faults via Operation Panel

To obtain accurate diagnostic data in 2026, you can use the FR-DU04 operation panel or the Parameter Unit (PU). These interfaces provide real-time status updates.

• Current Fault Display: When a protective function is activated, the operation panel automatically switches to alarm indication mode. Faults typically begin with “E.” (e.g., E.OC1, E.OV2) or “AL.” depending on the severity.
• Alarm History: The MD-AX520 memory stores the last 8 alarm records. You can recall these by pressing the MODE key to enter the “Alarm History” mode. This is vital for tracking intermittent issues like loose wiring or overheating.
• Status at Trip: By pressing the SET key while an alarm is displayed, you can cycle through the output voltage, current, and frequency values that were present at the exact moment of the fault.

General Troubleshooting Steps

Before attempting a reset, perform these essential safety checks:

1. Inspect the Regenerative Brake: If you see Overvoltage (E.OV) or Brake Duty (E.RB) errors, check the brake resistor for continuity and ensure the duty cycle isn’t too aggressive.
2. Check Power Stability: For Instantaneous Power Failure (E.IPF) or Undervoltage (E.UVT) faults, verify that the input power supply is stable and that there are no loose contactors in the control cabinet.
3. Verify Cooling: Ensure the drive’s internal cooling fan is running. Heat sink overheating (E.FIN) is a common cause of trips in enclosed cabinets.

Frequently Asked Questions (FAQ)

Q: What is the difference between an Alarm (AL) and a Protective Fault (E.)?

A: An Alarm (often displayed as flashing text like ‘OL’ for Overload or ‘RB’ for Regenerative Brake pre-alarm) warns of a threshold being reached but may allow the motor to continue running temporarily. A Protective Fault (codes starting with ‘E.’, such as E.OC1) is a critical failure that immediately shuts off the drive output to prevent hardware damage.

Q: How do I reset a Mitsubishi MD-AX520 fault?

A: You can reset the drive using one of the following methods:

• Press the STOP/RESET key on the operation panel.
• Cycle the main power supply (turn OFF, wait for the display to go dark, then turn ON).
• Trigger the external RES (Reset) signal via the terminal block wiring.

Q: What does the “E.PE” code mean?

A: E.PE indicates a “Parameter Error” or corruption in the storage device. This often happens if power is cut while writing parameters. You may need to perform an “All Parameter Clear” (Parameter 0 set to 1) and reprogram the drive, or replace the control board if the memory is physically damaged.