Unidrive M300 Drive Fault Codes (2026)

Unidrive M300 Drive Fault Codes (2026) – Complete 2026 troubleshooting guide for Nidec Control Techniques Unidrive M300. Learn to interpret mnemonic trip codes, troubleshoot dual STO safety errors, and use Connect software.

The Nidec Control Techniques Unidrive M300 is a robust general-purpose AC drive designed specifically for applications that require enhanced safety features. Building on the M200 platform, the M300 includes Dual Safe Torque Off (STO) terminals as standard, meeting SIL3/PLe safety standards. When the drive identifies a problem, it protects the system by initiating a “Trip” and displaying a mnemonic trip code—an abbreviation like OI.AC (Overcurrent) or It.AC (Motor Overload).

This 2026 guide is optimized to help you interpret these codes, manage the safety-related “Inhibit” states, and use advanced diagnostic software to minimize downtime.

Unidrive M300 Fault Codes Reference Table

Below is the reference list for the Unidrive M300. Please locate the mnemonic abbreviation appearing on your drive’s LED display for detailed troubleshooting steps.

Fault Code and Meaning	Cause and Remedy
C.Acc (185) NV Media Card Write fail	Cause: The drive was unable to access the NV Media Card. File corrupted, incomplete transfer, or card write failure. Remedy: Check NV Media Card is installed / located correctly. Replace the NV Media Card.
C.by (178) NV Media Card cannot be accessed (Busy)	Cause: Attempt made to access NV Media Card, but it is already being accessed by an Option Module. Remedy: Wait for the option module to finish accessing the NV Media Card and re-attempt the required function.
C.cPr (188) NV Media Card file/data is different to the one in the drive	Cause: A compare has been carried out and parameters on the NV Media Card are different to the drive. Remedy: Set Pr 00 to 0 and reset the trip. Check to ensure the correct data block on the NV Media Card has been used for the compare.
C.d.E (179) NV Media Card data location already contains data	Cause: Attempt made to store data on an NV Media Card in a data block which already contains data. Remedy: Erase the data in data location. Write data to an alternative data location.
C.dAt (183) NV Media Card data not found	Cause: Attempt made to access a non-existent file on the NV Media Card. Remedy: Ensure data file number is correct.
C.Err (182) NV Media Card data structure error	Cause: Error detected in the data structure on the card. Sub-trips indicate: (1) Missing folder/file structure, (2) Corrupted 000.DAT, (3) Duplicate file IDs. Remedy: Erase all the data block and re-attempt the process. Ensure the card is located correctly. Replace the NV Media Card.
C.Ful (184) NV Media Card full	Cause: Not enough space left on the card to create a data block. Remedy: Delete a data block or the entire NV Media Card to create space. Use a different NV Media Card.
C.OPt (180) NV Media Card trip; option module mismatch	Cause: Option module category differs between source and destination drives during transfer. Remedy: Ensure the correct option module is installed. Press the red reset button to acknowledge default values. This trip can be suppressed by setting Pr 00 to 9666 and resetting the drive.
C.Pr (175) NV Media Card data blocks not compatible	Cause: Drive Derivative or Product Type differs between source and target drives. Remedy: Use a different NV Media Card. Choose a file compatible between the source and target drives. This trip can be suppressed by setting Pr 00 to 9666 and resetting the drive.
C.rdo (181) NV Media Card has the Read Only bit set	Cause: Attempt to modify data on a read-only NV Media Card or data block. Remedy: Clear the read only flag by setting Pr 00 to 9777 and reset the drive.
C.rtg (186) Voltage/current rating mismatch	Cause: Current and/or voltage ratings are different between source and destination drives. Remedy: Reset the drive to clear the trip. Ensure that the drive rating dependent parameters have transferred correctly. This trip can be suppressed by setting Pr 00 to 9666 and resetting the drive.
C.SL (174) Option module file transfer has failed	Cause: Transfer of an option module file failed because the option module does not respond correctly. Remedy: Check option module installation. Retry transfer.
C.tyP (187) NV Media Card parameter set not compatible with current drive mode	Cause: The drive mode in the data block is different from the current drive mode, or operating mode is outside allowed range. Remedy: Ensure the destination drive supports the drive operating mode in the parameter file. Clear the value in Pr 00 and reset the drive. Ensure destination drive operating mode is the same as the source parameter file.
cL.A1 (28) Analog input 1 current loss	Cause: Current loss detected in current mode (falls below 3 mA). Remedy: Check control wiring is correct. Check control wiring is undamaged. Check the Analog Input 1 Mode (07.007). Ensure current signal is present and greater than 3 mA.
CL.bt (35) Trip initiated from the Control Word	Cause: Bit 12 set on control word in Pr 06.042 when control word is enabled. Remedy: Check the value of Pr 06.042. Disable the control word in Control Word Enable (Pr 06.043). Clear trip by setting bit 12 to zero.
Cur.c (231) Current calibration range	Cause: Current calibration range error. Remedy: Hardware fault – Contact the supplier of the drive.
Cur.O (225) Current feedback offset error	Cause: The current offset is too large to be trimmed. Remedy: Ensure that there is no possibility of current flowing in the output phases of the drive when the drive is not enabled. Hardware fault – Contact the supplier of the drive.
d.Ch (97) Drive parameters are being changed	Cause: User action or file system write is active while drive enabled (e.g. loading defaults, changing mode, transferring data). Remedy: Ensure the drive is not enabled when loading defaults. Ensure the drive is not enabled when changing drive mode. Ensure the drive is not enabled when transferring data from NV media card.
dcct (110) dcct reference out of range	Cause: The DCCT has caused a trip (size 5 upwards only). Remedy: Hardware fault – Contact the supplier of the drive.
dEr.E (246) Derivative file error	Cause: Derivative file error (Missing/invalid, hardware mismatch, or file changed). Remedy: Contact the supplier of the drive.
dEr.I (248) Derivative product image error	Cause: Error detected in the derivative product image (e.g., Divide by zero, Undefined trip, CRC failed). Remedy: Contact the supplier of the drive.
dESt (199) Two or more parameters are writing to the same destination	Cause: Destination parameters of two or more functions (Menus 7, 8, 9, 12 or 14) are writing to the same parameter. Remedy: Set Pr 00 to ‘dest’ or 12001 and check all visible parameters in all menus for parameter write conflicts.
dr.CF (232) Drive configuration (Hardware ID mismatch)	Cause: The hardware ID does not match the user software ID. Sub-trips: 1: Hardware ID mismatch (Size 5+). 2: Invalid hardware ID. 3: Hardware ID mismatch (Size 1-4). Remedy: Hardware fault – Contact the supplier of the drive.
EEF (31) Default parameters have been loaded	Cause: The EEF trip indicates that default parameters have been loaded due to memory corruption or hardware changes. Common Sub-trips: 1: Parameter database version changed. 2: CRC check failed. 3: Drive mode restored is outside allowed range. 9: Checksum on non-parameter area failed. Remedy: Default the drive and perform a reset. Allow sufficient time to perform a save before the supply to the drive is removed. If the trip persists – return drive to supplier.
Et (6) External trip initiated	Cause: An external trip has occurred via input or parameter. Sub-trip 3: External Trip (10.032) = 1. Can also be initiated by writing a value of 6 in Pr 10.038. Remedy: Check the value of Pr 10.032. Select ‘dest’ (or enter 12001) in Pr 00 and check for a parameter controlling Pr 10.032. Ensure Pr 10.032 or Pr 10.038 (= 6) is not being controlled by serial comms.
FAn.F (173) Fan fail	Cause: Cooling fan failure detected. This trip cannot be reset until 10s after trip initiation. Remedy: Check that the fan is installed and connected correctly. Check that the fan is not obstructed. Contact the supplier of the drive to replace the fan.
Fi.Ch (247) File changed	Cause: A file configuration change has been detected. Remedy: Power cycle the drive.
FI.In (237) Firmware incompatibility	Cause: The user firmware is incompatible with the power firmware. Remedy: Re-program the drive with the latest version of the drive firmware for the Unidrive M300, using Unidrive M Connect.
HF01 – HF04 Data processing error: CPU/Hardware	Cause: Internal hardware or memory management faults. HF01: CPU address error. HF02: DMAC address error. HF03: Bus fault. HF04: Usage fault. Remedy: Hardware fault – Contact the supplier of the drive.
HF07 – HF09 Data processing error: Watchdog/Interrupt	Cause: Internal processing faults. HF07: Watchdog failure. HF08: CPU Interrupt crash. HF09: Free store overflow. Remedy: Hardware fault – Contact the supplier of the drive.
HF11 Non-volatile memory comms error	Cause: Communication error with memory. Sub-trip 1: Comms error. Sub-trip 2: EEPROM size incompatible with firmware. Remedy: Sub-trip 1: Hardware fault – contact the supplier of the drive. Sub-trip 2: Re-program drive with compatible user firmware.
HF12 Main program stack overflow	Cause: Stack overflow occurred. Sub-trips indicate specific stack (background, timed, interrupt). Remedy: Hardware fault – Contact the supplier of the drive.
HF16 RTOS error	Cause: Real-time Operating System error occurred. Remedy: Hardware fault – Contact the supplier of the drive.
HF18 Internal flash memory has failed	Cause: Flash memory failed when writing option module parameter data. Sub-trip 1: Programming error. Sub-trip 2/3: Erase flash block failed. Remedy: Hardware fault – contact the supplier of the drive.
HF19 CRC check on the firmware has failed	Cause: Firmware corrupted. Drive is in bootloader mode. Remedy: Re-program the drive with latest control and power firmware using Unidrive M Connect. Hardware fault – Contact the supplier of the drive.
It.Ac (20) Output current overload timed out (I2t)	Cause: Motor thermal overload based on Motor Rated Current and Thermal Time Constant. Trip occurs when accumulator reaches 100%. Remedy: Ensure the load is not jammed / sticking. Check the load on the motor has not changed. Tune the motor rated speed parameter (Pr 05.008) (RFC-A mode only). Ensure the motor rated current is not zero.
It.br (19) Braking resistor overload timed out	Cause: Braking Resistor Thermal Accumulator (10.039) reached 100%. Remedy: Ensure the values entered in Pr 10.030, Pr 10.031 and Pr 10.061 are correct. Check resistor value and power rating. If an external thermal protection device is used, set Pr 10.030, Pr 10.031 or Pr 10.061 to 0.
LF.Er (90) Communication lost between power/control	Cause: Communications lost or excessive errors between power, control, or rectifier modules. Remedy: Hardware fault – contact the supplier of the drive.
no.PS (236) No power board	Cause: No communication between the power and control boards. Remedy: Hardware fault – contact the supplier of the drive.
O.Ld1 (26) Digital output overload	Cause: Total current drawn from AI Adaptor 24V or digital output exceeded limit. Sub-trip 1: Digital output or 24V load too high. Sub-trip 2: AI Adaptor 24V load too high. Remedy: Check total loads on digital outputs and 24 V. Check control wiring is correct. Check output wiring is undamaged.
O.SPd (7) Motor frequency exceeded threshold	Cause: Post-ramp reference or Estimated Frequency exceeded the Over Frequency Threshold (03.008). Remedy: Reduce the Frequency Controller Proportional Gain (03.010) to reduce overshoot (RFC-A mode). Check that a mechanical load is not driving motor. Reduce Current Controller Ki Gain (04.014).
Oht.C (219) Control stage over temperature	Cause: Control stage over-temperature detected. Remedy: Increase ventilation by setting Cooling Fan control (06.045) > 0.
Oh.dc (27) DC bus over temperature	Cause: DC bus component over temperature based on a software thermal model. The drive protection system estimates temperature based on output current and DC bus ripple. Remedy: Check the AC supply voltage balance and levels. Check DC bus ripple level. Reduce duty cycle. Reduce motor load. Check the output current stability. If unstable: Check motor map settings (Pr 05.006 – Pr 05.011). Disable slip compensation (Pr 05.027 = 0). Disable dynamic V to F operation (Pr 05.013 = 0). Select fixed boost (Pr 05.014 = Fixed). Disconnect load and complete rotating auto-tune.
Oht.I (21) Inverter over temperature based on thermal model	Cause: IGBT junction over-temperature detected based on software thermal model. Initiated when model reaches 145 °C. Remedy: Reduce the selected drive switching frequency. Ensure Auto-switching Frequency Change Disable (05.035) is set to Off. Reduce duty cycle. Increase acceleration / deceleration rates. Reduce motor load. Check DC bus ripple. Ensure all three input phases are present and balanced.
Oht.P (22) Power stage over temperature	Cause: Power stage over-temperature has been detected. Thermistor location is identified by the sub-trip. Remedy: Check enclosure / drive fans are still functioning correctly. Force the heatsink fans to run at maximum speed. Check enclosure ventilation paths. Check enclosure door filters. Increase ventilation. Reduce the drive switching frequency. Reduce duty cycle. Use S-ramp (Pr 02.006). Reduce motor load. Check the derating tables and confirm drive sizing.
OI.A1 (189) Analog input 1 over-current	Cause: Current input on analog input 1 exceeds 24 mA. Remedy: Check the signal source wiring and levels. Ensure the input is not shorted or receiving excess current.
OI.AC (3) Instantaneous output over current detected	Cause: The instantaneous drive output current has exceeded VM_DRIVE_CURRENT_MAX. Cannot be reset for 10s. Remedy: Increase acceleration/deceleration rate. If seen during auto-tune reduce the voltage boost. Check for short circuit on the output cabling. Check integrity of the motor insulation using an insulation tester. Ensure motor cable length is within limits. Reduce values in frequency loop gain parameters. Reduce values in current loop gain parameters.
OI.br (4) Braking IGBT over current detected	Cause: Over current detected in braking IGBT or braking IGBT protection activated. Cannot be reset for 10s. Remedy: Check brake resistor wiring. Check braking resistor value is greater than or equal to the minimum resistance value. Check braking resistor insulation.
OI.SC (228) Output phase short-circuit	Cause: Over-current detected on drive output when enabled. Possible motor earth fault. Remedy: Check for short circuit on the output cabling. Check integrity of the motor insulation using an insulation tester. Is the motor cable length within limits for the frame size?
OI.Sn (92) Snubber over-current detected	Cause: Over-current condition detected in the rectifier snubbing circuit. Remedy: Ensure the internal EMC filter is installed. Ensure motor cable length does not exceed maximum for selected switching frequency. Check for supply voltage imbalance. Check for supply disturbance (e.g., notching from DC drive). Check motor and cable insulation. Fit an output line reactor or sinusoidal filter.
Out.P (98) Output phase loss detected	Cause: Phase loss detected at drive output. (U, V, or W phase disconnected). Remedy: Check motor and drive connections. To disable the trip set Output Phase Loss Detection Enable (06.059) = 0.
OV (2) DC bus voltage exceeded peak level	Cause: DC bus voltage has exceeded the maximum limit for 15 seconds or instantaneous limit. Remedy: Increase deceleration ramp (Pr 04). Decrease the braking resistor value (staying above minimum). Check nominal AC supply level. Check for supply disturbances causing DC bus rise. Check motor insulation using an insulation tester.
P.dAt (220) Power system configuration data error	Cause: Error in the configuration data stored in the power system. Remedy: Hardware fault – Contact the supplier of the drive.
PAd (34) Keypad removed while receiving reference	Cause: Drive is in keypad mode and keypad has been removed or disconnected. Remedy: Re-install keypad and reset. Change Reference Selector (01.014) to select reference from another source.
Pb.bt (245) Power board in bootloader mode	Cause: Power board is in bootloader mode. Remedy: Send power board firmware file to reprogram the power board using Unidrive M Connect and power cycle drive.
Pb.Er (93) Communication lost between control and power	Cause: No communications between the control board processor and the power board processor. Remedy: Hardware fault – Contact the supplier of the drive.
Pb.HF (235) Power board HF	Cause: Power processor hardware fault. Remedy: Hardware fault – Contact the supplier of the drive.
Pd.S (37) Power down save error	Cause: Error detected in the power down save parameters saved in non-volatile memory. Remedy: Perform a 1001 save in Pr 00 to ensure that the trip doesn’t occur the next time the drive is powered up.
PH.Lo (32) Supply phase loss	Cause: The drive has detected an input phase loss, large supply imbalance, or DC bus ripple (ripple exceeds threshold). Remedy: Check the AC supply voltage balance and level at full load. Check the DC bus ripple level with an isolated oscilloscope. Check the output current stability. Check for mechanical resonance with the load. Reduce the duty cycle. Reduce the motor load. Disable the phase loss detection (set Pr 06.047 to 2).
PSU (5) Internal power supply fault	Cause: One or more internal power supply rails are outside limits or overloaded. Remedy: Remove the option module and perform a reset. If issue persists, there is a hardware fault – return the drive to the supplier.
r.All (227) RAM allocation error	Cause: An option module derivative image has requested more parameter RAM than is allowed. Remedy: Check option module parameters and menus (18, 20, 25, 29). Consult the module documentation for RAM usage.
r.b.ht (250) Hot rectifier/brake	Cause: Over-temperature detected on input rectifier or braking IGBT. Remedy: Increase ventilation by setting Cooling Fan Control (06.045) > 0.
rS (33) Measured resistance exceeded parameter range	Cause: The measured stator resistance during auto-tune exceeded the maximum possible value, or the value entered by user is invalid for the drive size. Remedy: Ensure the stator resistance falls within the range of the drive model. Check that the motor is not too small for the drive (ratio greater than 15:1). Check that a value has not been entered manually that exceeds the allowed range. Check motor cable and connections. Check integrity of motor stator winding (insulation tester). Check motor phase to phase resistance at drive and motor terminals. Select fixed boost mode (Pr 05.014 = Fd) and verify output waveforms. Replace the motor.
SCL (30) Control word watchdog timed out	Cause: The control word has been enabled and has timed out. Remedy: Once Pr 06.042 bit 14 has been changed from 0 to 1 to enable the watchdog, this must be repeated every 1s or an SCL trip is initiated. The watchdog is disabled when the trip occurs and must be re-enabled if required after reset.
SL.dF (204) Option module in slot 1 changed	Cause: The option module in slot 1 is a different type to that installed when parameters were last saved. Remedy: Turn off power, ensure the correct option module is installed, and re-apply power. Confirm installed module is correct, check parameters, and perform a user save in Pr mm.000.
SL.Er (202) Option module in slot 1 detected a fault	Cause: The option module has detected an error. Remedy: See relevant option module User Guide for details of the trip.
SL.HF (200) Option module 1 hardware fault	Cause: Hardware fault generated by the drive regarding the module (e.g., ID not recognized, memory insufficient, connection lost). Remedy: Ensure the option module is installed correctly. Replace the option module. Replace the drive.
SL.nF (203) Option module in slot 1 removed	Cause: The option module has been removed since the last power up. Remedy: Ensure the option module is installed correctly. Re-install the option module. To confirm removal is intentional, perform a save function in Pr 00.
SL.tO (201) Option module watchdog service error	Cause: The option module started the watchdog function and then failed to service it correctly. Remedy: Replace the option module.
So.St (226) Soft start relay failed	Cause: Soft start relay failed to close or soft start monitoring circuit failed (Sub-trips: 1=Soft-start failure, 2=DC bus capacitor failure). Remedy: Hardware fault – Contact the supplier of the drive.
St.HF (221) Hardware trip during last power down	Cause: A hardware trip (HF01 – HF18) occurred and the drive has been power cycled. Remedy: Enter 1299 in Pr 00 and press reset to clear the trip.
Sto (234) No Safe Torque Off board fitted	Cause: STO board not fitted correctly. Remedy: Hardware fault – Contact the supplier of the drive.
th (24) Motor thermistor over-temperature	Cause: Motor thermistor connected to terminal 14 indicated over-temperature. Remedy: Check motor temperature. Check threshold level (Pr 07.048). Check thermistor continuity.
th.br (10) Brake resistor over temperature	Cause: Hardware based braking resistor thermal monitoring detected overheat. Remedy: Check brake resistor wiring. Check brake resistor value is greater than or equal to minimum resistance value. Check braking resistor insulation.
tH.Fb (218) Internal thermistor has failed	Cause: Internal thermistor failed (open circuit or short circuit). Remedy: Hardware fault – Contact the supplier of the drive.
thS (25) Motor thermistor short circuit	Cause: Motor thermistor connected to terminal 14 is short circuit or low impedance (<50 Ω). Remedy: Check thermistor continuity. Replace motor / motor thermistor.
tun.S (18) Autotune test stopped before completion	Cause: The drive was prevented from completing an autotune test, because either the drive enable or the drive run were removed. Remedy: Check the drive enable signal (Terminal 31 & 34 on size 1 to 4 or terminals 31 & 35 on size 5 to 9) were active during the autotune. Check the run command was active in digital input 3 or 4 state (Pr 08.003 or Pr 08.004) during the autotune.
tun.1 (11) Required speed could not be reached	Cause: The drive has tripped during an autotune. The cause of the trip can be identified from the sub-trip number: Sub-trip 2: The motor did not reach the required speed during rotating autotune or mechanical load measurement. Remedy: Ensure the motor is free to turn i.e. mechanical brake is released. Ensure Mechanical Load Test Level (05.021) is set correctly.
tun.3 (13) Measured inertia has exceeded the parameter range (RFC-A mode only)	Cause: The drive has tripped during a rotating autotune or mechanical load measurement test. The cause of the trip can be identified from the associated sub-trip number: Sub-trip 1: Measured inertia has exceeded the parameter range during a mechanical load measurement. Sub-trip 3: The mechanical load test has been unable to identify the motor inertia. Remedy: Check motor cable wiring is correct.
U.OI (8) User OI ac	Cause: A U.OI trip is initiated if the output current of the drive exceeds the trip level set by User Over Current Trip Level (04.041). Remedy: Check load conditions and User Over Current Trip Level settings.
U.S (36) User Save error / not completed	Cause: The U.S trip indicates that an error has been detected in the user save parameters saved in non-volatile memory. For example, following a user save command, if the power to the drive was removed when the user parameters were being saved. Remedy: Perform a user save in Pr 00 to ensure that the trip doesn’t occur the next time the drive is powered up. Ensure that the drive has enough time to complete the save before removing the power to the drive.
UP.uS (96) Trip generated by an onboard user program	Cause: This trip can be initiated from within an onboard user program using a function call which defines the sub-trip number. Remedy: Check the user program.
UPrG (249) Onboard user program error	Cause: An error has been detected in the onboard user program image. The sub-trip indicates the reason for the trip: Sub-trip Reason / Comments 1 Divide by zero. 2 Undefined trip. 3 Attempted fast parameter access set-up with non-existent parameter. 4 Attempted access to non-existent parameter. 5 Attempted write to read-only parameter. 6 Attempted an over-range write. 7 Attempted read from write-only parameter. 30-34 Image failure (CRC, RAM, OS call, ID code, or User Program Number). 40 Timed task not completed in time (Enable 11.047 reset to zero). 41 Undefined function called. 52 Customizable menu table CRC check failed. 53 Customizable menu table changed (Defaults loaded). 80-81 Image not compatible with control board/serial number. 100-104 Pointer/Array access violations or unknown data types. 200 “Divide” service with denominator of zero. Remedy: Review the onboard user program logic corresponding to the sub-trip. Ensure image compatibility with the control board. Check for array bounds violations or invalid parameter access in code.
HFxx Internal faults	Cause: The main drive processor has detected an irrecoverable error. Internal problems. Remedy: These indicate internal problems and cannot be reset. All drive features are inactive. Power cycle the drive.
{St.HF} Stored HF trip	Cause: A non-permanent HF trip occurred previously and the drive was power cycled. Remedy: This trip cannot be cleared unless 1299 is entered into Parameter 00 and a reset is initiated.
{SL.HF} Non-resettable trips (218-247)	Cause: Hardware fault detected. Remedy: These trips cannot be reset. Check hardware / contact support.
{EEF} Volatile memory failure	Cause: Data integrity issue in volatile memory. Remedy: This can only be reset if Parameter 00 is set to 1233 or 1244, or if Load Defaults (11.043) is set to a non-zero value.
{PSU} Internal 24V	Cause: Issue with Rectifier 24V supply. Remedy: Check internal power supply conditions (Priority 4).
{OI.AC}, {OI.br}, {FAn.F} Trips with extended reset times	Cause: Over-current or fan fault conditions. Remedy: These trips cannot be reset until 10 s after the trip was initiated.
{PH.Lo}, {Oh.dc} Phase loss and d.c. link power circuit protection	Cause: Input phase loss or DC link issue. The drive attempts to stop the motor before tripping. Remedy: Check Action On Trip Detection (10.037). Check input supply and DC link.
br.res Brake resistor overload	Cause: Braking Resistor Thermal Accumulator (10.039) in the drive has reached 75.0 % of the value at which the drive will trip. Remedy: Check braking load and resistor sizing.
OV.Ld Motor Protection Accumulator	Cause: Motor Protection Accumulator (04.019) has reached 75.0 % of the trip value and load is >100%. Remedy: Reduce motor load.
d.OV.Ld Drive over temperature	Cause: Percentage Of Drive Thermal Trip Level (07.036) is greater than 90 %. Remedy: Check cooling fans, ambient temperature, and drive ventilation.
tuning Autotune in progress	Cause: The autotune procedure has been initialized and is running. Remedy: Wait for autotune to complete.
LS Limit switch active	Cause: Indicates that a limit switch is active and is causing the motor to be stopped. Remedy: Check machine position and limit switches.
Opt.AI Option slot alarm	Cause: Alarm generated by the module in the option slot. Remedy: Check option module status.
Lo.AC Low voltage mode	Cause: Drive is operating in Low AC mode. See Low AC Alarm (10.107). Remedy: Check supply voltage settings.
I.AC.Lt Current limit active	Cause: Drive has reached current limit. See Current Limit Active (10.009). Remedy: Check mechanical load and current limit parameters.
24.LoSt 24V Backup not present	Cause: 24V backup supply is missing. See 24V Alarm Loss Enable (11.098). Remedy: Check external 24V backup supply.
inh Inhibit	Cause: The Safe Torque Off (STO) signal is not applied to terminals or Pr 06.015 is set to 0. Remedy: Apply STO signal to enable the drive (Output stage: Disabled).
rdy Ready	Cause: Drive enable is active, but the drive inverter is not active because the final drive run is not active. Remedy: Give run command to start (Output stage: Disabled).
Stop Stopped	Cause: The drive is stopped / holding zero speed. Remedy: Normal operation (Output stage: Enabled).
S.Loss Supply loss	Cause: Supply loss condition has been detected. Remedy: Check incoming power supply (Output stage: Enabled).
dc.inj DC injection	Cause: The drive is applying dc injection braking. Remedy: Wait for braking to complete (Output stage: Enabled).
Er Error	Cause: The drive has tripped and is no longer controlling the motor. The trip code appears in the display. Remedy: Troubleshoot specific trip code (Output stage: Disabled).
UV Under voltage	Cause: The drive is in the under voltage state either in low voltage or high voltage mode. Remedy: Check DC bus voltage and supply (Output stage: Disabled).
HEAt Heat	Cause: The motor pre-heat function is active. Remedy: Normal operation if configured (Output stage: Enabled).
PS.LOAD Waiting for power stage	Cause: The drive is waiting for the processor in the power stage to respond after power-up. Remedy: Wait for initialization.
LOAD OPtion Waiting for option module	Cause: The drive is waiting for the option module to respond after power-up. Remedy: Wait for initialization.
UPLOAD Loading parameter database	Cause: At power-up, parameter database is updating because an Option module changed. Remedy: Wait for data transfer between drive and option module.
LOAD.I Bootloading drive firmware	Cause: The drive is waiting for the bootloader file to be transferred to the processor. Remedy: Wait for firmware load to complete.

Safety Note: Because the M300 features Dual STO, a trip may be triggered if there is a discrepancy between the two safety channels. Always check your safety relay or emergency stop circuit.

---

How to Read M300 Faults via “Connect” Software

In 2026, the most effective way to troubleshoot the Unidrive M300 is through the Control Techniques Connect software. This tool provides a clear, high-resolution view of the drive’s parameters that the basic keypad cannot provide.

• Onboard RS485 & SI-Slot: Connect via the built-in RS485 port or through an optional Ethernet module (like the SI-Ethernet) in the option slot.
• Trip Log (Menu 10): The M300 maintains a record of the last 10 trips in Parameters 10.020 to 10.029. “Connect” allows you to view these with precise run-hour timestamps.
• Sub-trip Information: Many trips include a sub-trip string (accessible in Parameter 10.070) which identifies the specific hardware module or internal process that failed.
• Diagnostic Dashboard: Use the software to monitor real-time output frequency, current, and the status of both STO channels simultaneously to identify intermittent safety circuit drops.

Frequently Asked Questions (FAQ)

Q: What is the difference between a Trip and a Warning on the M300?

A: A Trip (Solid code) is a protective shutdown that stops the motor. A Warning (Flashing code) indicates the drive is approaching a limit—such as br.res (Braking Resistor Overload)—but will keep running unless the condition worsens into a hard trip.

Q: How do I reset a Unidrive M300 fault?

A: Once the root cause is resolved, you can reset the drive via:

• Pressing the Red STOP/RESET button on the keypad.
• Cycling the Main Power (Wait for the display to turn off fully).
• Toggling a Digital Input assigned to the “Reset” function (Parameter 10.033).
• Sending a reset command via Modbus RTU or an optional fieldbus card.

Q: What does “inh” (Inhibit) mean?

A: “inh” is not a fault code. It means the drive is Inhibited because one or both of the Safe Torque Off (STO) terminals are not receiving a 24V signal. The drive is healthy but is being prevented from starting by the external safety circuit.

Q: Where is the fault history stored?

A: The trip history is stored in the drive’s non-volatile memory in Menu 10. This log is retained even if the drive is powered down for maintenance, and the most recent trip is always found in parameter 10.020.

Q: My drive shows “UV,” is it broken?

A: “UV” stands for Under Voltage. This is normal when the drive is powered down. However, if it happens during a run, it indicates the DC bus has dropped below the trip level, likely due to a power sag or a dropped input phase.