Mitsubishi MD-CX520 Fault Codes List

Complete maintenance guide for Mitsubishi MD-CX520 drive units. Learn to interpret “E.” trip codes, diagnose MELIPM motor issues, and use the FR-PU parameter unit for troubleshooting.

MD-CX520 Fault Codes Reference Table

Below is the reference list for identifying drive unit specific alarms:

Mitsubishi MD-CX520 Fault Codes List

Mitsubishi MD-CX520 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.

Remedy:
  • Increase the acceleration time.
  • Change to the S-pattern acceleration/deceleration.
  • Check the motor winding resistance.
  • Check the connection cables for damage.
  • Check if drive unit capacity matches motor capacity.
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.

Remedy:
  • Check the motor winding resistance.
  • Check the connection cables for damage.
  • Check if drive unit capacity matches motor capacity.
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 operation. The mechanical brake of the motor operates too early.

Remedy:
  • Increase the deceleration time.
  • Change to the S-pattern acceleration/deceleration.
  • Check the motor winding resistance.
  • Check the connection cables for damage.
  • Delay the brake operation timing.
E.OV1

Acceleration time overvoltage		 Cause:
Surge compounded with power during acceleration operation. The regenerative energy handling capability is insufficient.

Remedy:
  • Install a reactor.
  • Fit a surge suppressor or like to the surge source.
  • Fit the regenerative brake option.
E.OV2

Constant speed-time overvoltage		 Cause:
Surge compounded with power during constant speed operation. Load increased suddenly, activating the stall prevention function. The regenerative energy handling capability is insufficient.

Remedy:
  • Install a reactor.
  • Fit a surge suppressor or like to the surge source.
  • Fit the regenerative brake option.
E.OV3

Deceleration time overvoltage		 Cause:
The deceleration torque is beyond the drive unit capability. The regenerative energy handling capability is insufficient. Surge compounded with power during deceleration operation or stop.

Remedy:
  • Increase the deceleration time.
  • Change to the S-pattern acceleration/deceleration.
  • Fit the regenerative brake option.
  • Install a reactor.
  • Fit a surge suppressor or like to the surge source.
E.THT / E.THM

Electronic thermal O/L relay		 Cause:
The motor and drive unit are operated under overload.

Remedy:
  • Reduce the load.
  • Increase the capacities of the motor and drive unit.
E.UVT

Undervoltage		 Cause:
An instantaneous voltage drop in the AC input power supply. The AC input power supply voltage is insufficient.

Remedy:
  • Check AC input power supply.
  • Do not switch AC input power off while motor is running at high speed.
E.FIN

Fin overheat		 Cause:
Heatsink temperature is higher than the permissible value. The surrounding air temperature exceeded permissible limits. The cooling fan failed.

Remedy:
  • Check cooling fan operation.
  • Check ambient temperature.
E.OHT

External failure		 Cause:
An external relay or another relay, which is connected to the terminal where the OH signal is assigned, turned ON.

Remedy:
  • Check the external device causing the OH signal.
  • Refer to Control Circuit Contact Input Terminal Functions.
E.PUE

PU disconnection		 Cause:
A connection fault of the operation panel or the parameter unit occurred. RS-485 communication was interrupted.

Remedy:
  • Check connection of the operation panel.
  • Check RS-485 communication settings (Pr. 121).
E.OLT

Stall prevention stop		 Cause:
Motor overload activated the stall prevention function consecutively, stopping the motor.

Remedy:
  • Reduce the load.
  • Increase the capacities of the motor and drive unit.
E.LF

Open loss		 Cause:
One of the output phases U, V, or W was lost.

Remedy:
  • Check wiring of output phases.
  • Check for phase loss during motor driving.
E.CPU

CPU fault		 Cause:
Malfunction of CPU.

Remedy:
  • If error persists after reset, replace the drive unit.
E.PE

Storage device fault		 Cause:
The storage device failed.

Remedy:
  • If error persists 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 after reset.
  • Do not leave power on to prevent overheating brake resistor.
OL

Overload 1 / Overload 2		 Cause:
Overcurrent or Main circuit DC voltage too high. (Stall prevention activated).

Remedy:
  • See remedies for Overcurrent (E.OCx) or Overvoltage (E.OVx).
  • Reduce load.
FN

Fan failure		 Cause:
Cooling fan fault.

Remedy:
  • Replace the cooling fan.
Err

Operation error		 Cause:
Parameter changed during External operation. Value outside range. RES signal remains ON. Connection fault.

Remedy:
  • Check the fitting status of the panel.
  • Check parameter settings and operation mode.
PS

Emergency stop		 Cause:
The STOP/RESET key was pressed to make a stop during External operation.

Remedy:
  • Reset cannot be made via normal method. Refer to Pr. 75.
 

How to Read MD-CX520 Faults

To get the most accurate diagnostic data in 2026, you generally rely on the FR-PU04 / FR-PU07 Parameter Unit or the FR Configurator software if a communication option is installed.

  • Code History: The MD-CX520 retains a history of the last 8 alarm codes. Accessing the “Alarm History” menu via the PU unit allows you to see if a fault like “E.THT” (Motor Overload) is recurring.
  • Status at Trip: You can view the output frequency, current, and voltage at the exact moment the alarm occurred. This is critical for distinguishing between a mechanical jam (high current) and an electrical noise issue.
  • Digital Inputs: Check the status of input terminals (STF, STR) via the monitor mode to ensure external control signals aren’t inadvertently triggering a stop or safety trip.

Frequently Asked Questions (FAQ)

Q: What does the fault “E.OC1” mean on an MD-CX520?
A: E.OC1 indicates an Overcurrent during acceleration. This is common in pump/fan applications if the ramp-up time is too short for the load’s inertia, or if there is a short circuit in the motor cables.

Q: Can I run a standard induction motor with the MD-CX520?
A: No. The MD-CX520 is specifically designed for MELIPM (Permanent Magnet) motors. Connecting a standard induction motor can cause immediate fault trips or motor damage due to the different control algorithms.

Q: How do I reset the drive after a fault?
A: You can reset the drive by pressing the RESET key on the connected parameter unit, cycling the main power, or toggling the “RES” (Reset) terminal on the I/O block.

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