Mitsubishi FR-B-750 Fault Codes List

Access the complete reference guide for Mitsubishi FR-B-750 drive fault codes. Learn to diagnose E.OCx errors, motor matching faults etc.

This guide provides the most updated reference for 2026 to help you troubleshoot these critical safety drives.

FR-B-750 Fault Codes Reference Table

Use the search bar below or browse the table to identify your specific error code.

Mitsubishi FR-B-750 Fault Codes List

Mitsubishi FR-B-750 Fault Codes List

Fault Code and Meaning Cause and Remedy
E.OC1 Overcurrent shut-off during acceleration Cause: When the inverter output current reaches or exceeds approximately 200% of the rated current during acceleration.
Check points: Check for sudden acceleration, output short circuit, or if main circuit power (R, S, T) is supplied. Remedy:
  • Increase the acceleration time.
  • If "E.OC1" is always lit at a start, disconnect the motor once and then start the inverter. If "E.OC1" is still lit, contact your sales representative.
  • Supply main circuit power (R, S, T).
E.OC2 Overcurrent shut-off during constant speed Cause: When the inverter output current reaches or exceeds approximately 200% of the rated current during constant speed.
Check points: Check for sudden load change or output short circuit. Remedy:
  • Keep load stable.
E.OC3 Overcurrent shut-off during deceleration Cause: When the inverter output current reaches or exceeds approximately 200% of the rated current during deceleration.
Check points: Check for sudden speed reduction, output short circuit, or too fast operation of motor's mechanical brake. Remedy:
  • Increase the deceleration time.
  • Check mechanical brake operation.
E.OV1 Regenerative overvoltage shut-off during acceleration Cause: Regenerative energy causes the inverter's internal main circuit DC voltage to reach or exceed the specified value. It may also be activated by a surge voltage generated in the power supply system.
Check points: Check for too slow acceleration. Remedy:
  • Decrease the acceleration time.
E.OV2 Regenerative overvoltage shut-off during constant speed Cause: Regenerative energy causes internal main circuit DC voltage to exceed specified value (or surge voltage).
Check points: Check for sudden load change. Remedy:
  • Keep load stable.
  • Use the brake unit or power return converter (FR-RC) as required.
E.OV3 Regenerative overvoltage shut-off during deceleration or stop Cause: Regenerative energy causes internal main circuit DC voltage to exceed specified value (or surge voltage).
Check points: Check for sudden speed reduction. Remedy:
  • Increase the deceleration time. (Set the deceleration time which meets load GD²).
  • Decrease the braking duty.
  • Use the brake unit or power return converter (FR-RC) as required.
E.THM Motor overload shut-off (electronic overcurrent protection) Cause: Motor overheat due to overload or reduced cooling capability during constant-speed operation. (Pre-alarm occurs at 85% of preset value).
Check points: Check the motor for use under overload. Remedy:
  • Reduce the load weight.
  • For a constant-torque motor, change the Pr. 71 setting to the constant-torque motor setting.
E.THT Inverter overload shut-off (electronic overcurrent protection) Cause: Current of more than 150% of the rated output current flows. Inverse-time characteristics activate protection to protect output transistors.
Check points: Check the motor for use under overload. Remedy:
  • Reduce the load weight.
E.IPF Instantaneous power failure protection Cause: A power failure occurred for longer than 15ms.
Check points: Find the cause of instantaneous power failure occurrence. Remedy:
  • Remedy the instantaneous power failure.
  • Prepare a backup power supply for instantaneous power failure.
  • Set the function of automatic restart after instantaneous power failure.
E.UVT

Undervoltage protection		 Cause:
Power supply voltage reduces below 150V (about 300V for the 400V class). Large-capacity motor start causing voltage dip. Jumper not connected across P-P1.

Remedy:
  • Check the power supply system equipment.
  • Connect a jumper or DC reactor across terminals P-P1.
E.FIN

Fin overheat		 Cause:
Cooling fin overheats (sensor actuated). Too high ambient temperature or cooling fin clogging.

Remedy:
  • Set the ambient temperature to within the specifications.
  • Clean the cooling fins.
E.GF

Output side ground fault overcurrent protection		 Cause:
Ground fault overcurrent flows due to a ground fault in the inverter's output side (load side). Check for ground fault in motor or connection cable.

Remedy:
  • Remedy the ground fault portion.
E.OHT

OH Fault (External thermal relay operation)		 Cause:
External thermal relay or internal motor temperature relay switched on (contacts open). Motor overheating. Incorrect setting of Pr. 180 to Pr. 186.

Remedy:
  • Reduce the load and operating duty.
  • Check Pr. 180 to Pr. 186 settings.
E.BE

Br. Cct. Fault (Brake transistor alarm detection)		 Cause:
Brake circuit fault due to damaged brake transistors. Load GD² is too high or improper braking usage frequency.

Remedy:
  • Change the inverter.
  • Reduce load GD².
E.OLT

Stall prevention (Still Prev STP)		 Cause:
Running frequency fell to 0 because stall prevention was activated. Motor used under overload.

Remedy:
  • Reduce the load weight.
E.OPT

Option Fault		 Cause:
Dedicated option setting error or connection fault. High power factor converter connection set but AC power supplied to R, S, T. Loose connector.

Remedy:
  • Connect the plugged-in option securely.
  • Contact your sales representative.
E.OP1 to OP3

Option slot alarm 1 to 3		 Cause:
Functional alarm in the plug-in option (e.g. communication alarm). Wrong option function setting or operation.

Remedy:
  • Confirm the option function setting, etc.
E.PE

Corrupt Memory (Parameter storage device alarm)		 Cause:
Fault in the E²PROM device. Too many parameter write times.

Remedy:
  • Contact your sales representative.
E.PUE

PU Leave Out (Parameter unit disconnection)		 Cause:
Communication between inverter and PU suspended/disconnected. Loose fitting of DU or PU. Pr. 75 setting issue.

Remedy:
  • Fit the DU and PU securely.
E.RET

Retry No Over (Retry count exceeded)		 Cause:
Operation could not be resumed within the set number of retries.

Remedy:
  • Eliminate the cause of the error preceding this indication.
E.LF

Output phase failure protection		 Cause:
One of the three phases (U, V, W) on the output side is open. Rated motor current is extremely lower than rated inverter current. Loose wiring.

Remedy:
  • Check the cables properly.
  • Check the setting of Pr. 251.
E.CPU

CPU Fault		 Cause:
Arithmetic operation of the built-in CPU does not end within a predetermined period.

Remedy:
  • Make connection securely.
  • Contact your sales representative.
E.6 / E.7

Fault 6 / Fault 7 (CPU error)		 Cause:
Arithmetic operation of peripheral circuit failed or receive data error in built-in CPU. Loose connector.

Remedy:
  • Make connection securely.
  • Contact your sales representative.
E.P24
24VDC power output short circuit		 Cause: Check for a short circuit in the PC terminal output.

Remedy:
Remedy the short circuit portion.
E.CTE
Operation panel power supply short circuit		 Cause: Check for a short circuit in the PU connector cable.

Remedy:
Check the PU and cable.
E.MB1 to 7
Brake sequence error		 Cause: Find the cause of alarm occurrence.

Remedy:
Perform parameter setting check and wiring properly.
FN
Fan fault		 Cause: Check the cooling fan for a fault.

Remedy:
Change the fan.
OL
Stall prevention (Over current)		 Cause: Check the motor for use under overload.

Remedy:
The acceleration time may change. Increase the stall prevention operation level using Pr. 22 "stall prevention operation level" or make the stall prevention inoperative using Pr. 156 "stall prevention operation selection".
oL
Stall prevention (overvoltage)		 Cause: Check for sudden speed reduction.

Remedy:
The deceleration time may change. Increase the deceleration time using Pr. 8 "deceleration time".
PS
PU stop		 Cause: Check for a stop made by pressing the STOP key of the operation panel.

Remedy:
Refer to page 98.
Err.
Operation Error		 Cause: This alarm appears if:
  • The RES signal is on.
  • You attempted to set any parameter value in the external operation mode.
  • You attempted to change the operation mode during operation.
  • You attempted to set any parameter value outside its setting range.
  • Normal communication cannot be made between the PU and inverter.
  • You attempted to make parameter setting when Pr. 77 "parameter write disable selection" has been set to disable parameter write.

Remedy:
Perform operation correctly.
 

How to Read FR-B-750 & Auto Tuning

Because the FR-B-750 series operates in hazardous areas, the inverter itself is usually located in a safe cabinet away from the motor. Diagnostics rely heavily on the Parameter Unit (PU) and strict motor matching:

  • Parameter Unit (FR-B-750): The FR-B-750 series typically uses these handheld units.
    • Fault History: Press the Alarm History key to view the last 8 faults. Crucially, check the Output Voltage and Current at the time of the trip to verify if the explosion-proof motor was actually overloaded or if the drive miscalculated the vector model.
  • Offline Auto Tuning: The FR-B-750 must be tuned to the specific explosion-proof motor it is driving.
  • Motor Compatibility Check: The FR-B-750 cannot drive standard induction motors effectively. If you see persistent E.THT (Motor Overload) faults, verify that a standard motor wasn’t accidentally installed in the hazardous zone.

Frequently Asked Questions (FAQ)

Q: How do I reset a Mitsubishi FR-B-750 fault?

A:

  1. Press the RESET key on the Parameter Unit (PU).
  2. Toggle the RES (Reset) terminal input (ensure the safe area control cabinet is accessible).
  3. Power cycle the drive. Warning: In hazardous areas, ensure all safety protocols are followed before re-energizing equipment.

Q: Why do I get “E.BE” (Brake Transistor Alarm)?

A: This implies the internal brake transistor is defective or the external braking resistor is overheating. In explosion-proof applications, regenerative braking energy must be managed carefully to avoid creating heat sources in the hazardous zone. Check the thermal relay on the external brake resistor.

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