Fanuc Servo Amplifier Alarm Codes: Comprehensive Guide to Alarms & Troubleshooting

In CNC systems, the servo amplifier is a critical component. This article focuses on the Fanuc servo amplifier alarm codes you may encounter on a Fanuc α-series servo amplifier and walks through systematic troubleshooting methods. The goal: when you see a code, you know what it means and what to do next.

Fanuc servo amplifier alarm codes

1. Fan / Cooling Alarm (LED displays “1 ALM”)

Cause analysis: The servo amplifier’s cooling fan is either overheated, dirty/clogged, or failed altogether.

Solution steps:

  1. Check if the fan (typically located on the top of the servo unit) is running. If no airflow or the fan doesn’t spin: remove the fan and check for heavy oil/grease contamination. Clean with gasoline or alcohol, reinstall, test again. If still non-functional → replace the fan.
  2. Replace the small interface board (auxiliary board) if applicable.
  3. Remove the main control board and use a multimeter to measure the continuity from the fan socket to the CN1 connector (which connects to the small interface board). Check for broken wiring.

2. DC Link Low-Voltage Alarm (LED displays “2 ALM”)

Cause analysis: The servo amplifier detects that the DC link voltage (≈300 V) is too low. Possible reasons: rectified DC from AC input is too low; external AC input voltage is too low; the alarm-detection circuit is faulty.

Solution steps:

  1. Measure the three-phase AC input voltage (note: since the MCC contactor may have already opened because of the alarm, you might need to measure upstream of the MCC).
  2. Check whether the MCC contactor contacts are worn or not making good connection.
  3. On the main control board, check if the detection resistor is burnt open.
  4. If all above are normal, consider replacing the servo amplifier unit.

3. Power-Unit Low-Voltage Alarm (LED displays “5 ALM”)

Cause analysis: The servo amplifier detects that the auxiliary power supply voltage is too low — meaning the control power inside the amplifier is insufficient, or the detection circuit is faulty.

Solution steps:

  1. Measure the three-phase AC input to the power unit (again upstream of MCC if required).
  2. Verify the MCC contactor is making proper contact.
  3. Check the detection resistor on the main control board for burn-out.
  4. Replace the power unit or the servo amplifier unit.

4. Abnormal Current Alarm (LED displays “8, 9, A, B, C, D, E”)

Cause analysis: The servo amplifier has detected an abnormal current. This may result from a short in the main power circuit, a fault in the drive/control circuit, or a failure in the detection circuit. The letter/number corresponds to axis Fanuc servo amplifier alarm codes:

  • “8” → L axis
  • “9” → M axis
  • “A” → N axis
  • “B” → LM two-axes
  • “C” → LN two-axes
  • “D” → MN two-axes
  • “E” → LMN three-axes

Solution steps:

  1. Check whether the IPM (Intelligent Power Module) is burnt. Most of these alarms are due to IPM short-circuit. Using a multimeter set to diode test, measure between U, V, W phases and +/- bus. If the drop is ≈ 0 Ω, the module is burnt. If so: remove cover, unbolt fixed module screws, replace IPM.
  2. If alarm occurs immediately on power-on, swap the interface board with another unit. If fault transfers, the interface board is defective.
  3. Swap the control board with another unit. If fault transfers, replace or send the control board to Fanuc for repair.
  4. With the unit powered off, disconnect the motor power line and test again (for gravity axis, ensure machine side is locked to prevent drop). If alarm clears, the fault may be in the motor or power cable.
  5. Swap motor output/feedback/command lines with another axis. If feedback line of servo follows into the amplifier (Servo B type wiring), swap that too. If the Fanuc servo amplifier alarm codes remains unchanged → fault is external to the amplifier (motor, cable, wiring). Use insulation tester to check motor and power cable. Use multimeter to check feedback line, command line, axis control board wiring.
  6. Check the servo parameter settings in the system for correct values.
  7. If the alarm only appears after the unit has been powered for some time and disappears after a downtime, the IPM may be overheating: check if the load is too high or the cooling is inadequate.
  8. If all the above fail and there isn’t an identical axis to swap (for example on a lathe X and Z axes are dissimilar), you can: disconnect the motor power cable. If still no result, connect power cable and swap servo parameters between two axes, then judge again.
fanuc servo amplifier alarm codes

5. IPM Alarm (LED displays “8. , 9. , A. , B. , C. , D. , E.”)

Cause analysis: Note that when the LED displays e.g. “8.” (with a small dot next to it) — this indicates the IPM module alarm for that axis (versus plain “8” which is high-current). Examples: 8. → L-axis IPM alarm; 9. → M-axis IPM alarm; A. → N-axis IPM alarm.

Solution steps:

  1. If alarm persists: replace IPM module or small interface board — typically multimeter cannot locate this fault directly.
  2. If the alarm is time-dependent (appears only after some runtime, clears after idle) → suspect IPM overheating; verify loading and cooling.
  3. Swap cables (motor power, command, feedback) with another axis as described above. If Fanuc servo amplifier alarm codes remains, check motor, power cable, feedback wiring, command wiring. Also swap axis control boards.
  4. Same as step 8 of section 4: if no identical axis available for swapping, disconnect motor power line first then proceed to swapping parameters between axes.

6. “Servo VRDY OFF” Alarm (Ready signal not received) (Displays: 0, 16/18/0I → 401)

Cause analysis: During system startup self-check, assuming no E-stop or other alarms, the CNC control issues *MCON signal to all servo amplifier units. The servo units upon receiving *MCON energize the main contactor, the power-unit engages, LED changes from “­­” to “00”, and then the amplifier sends back *DRDY signal to the system. If the system does not receive *DRDY within the specified time, it issues this alarm and cuts off all *MCON signals to the axes. Therefore, all links in this path (CNC → command bus → interface boards → servo amplifier → contactor → *DRDY feedback) can be a fault location.

Solution steps:

  1. Inspect all connectors for loose or poor contact: control board to main circuit, power unit to amplifier, spindle unit connection.
  2. Observe whether the LED displays at all. If no display → the board may not be powered, or the power-unit or power circuit is faulty. Check 24 V output from power-unit to the amplifier; inspect fuse and power circuit on the control board. If beyond your repair capability → send to Fanuc for repair.
  3. Check external AC voltages: for the power-unit 3-phase 200 V input (R,S,T) and single-phase 200 V input.
  4. On control board, measure all DC voltages. If abnormal: check the fuse and the board’s power circuit for burn-out.
  5. Watch the power unit’s LED: does it change from “­­” to “00” (i.e., contactor engaged) and then back to “­­”? Or does it remain “­­” (never engages)?
    • If it engages then drops, the power-unit may be faulty.
    • If it never engages, likely wiring or power-unit issue: check interlock wiring between units.
  6. Check the *ESP (E-stop) and *MCC contactor circuit of the power-unit. If either circuit is open, the LED stays “­­”. *ESP should be short circuit when released; *MCC should be in series with the contactor coil and AC input.
  7. Observe the servo amplifier LED: after it changes to “00” (contactor engaged), check whether all servo amplifier axes display a “0” (ready) or one stays as “-”. If one axis remains “-” and the rest OK, that axis’ amplifier is faulty: check the relay inside the amplifier; if replacing the relay doesn’t work → replace the unit’s interface board.
  8. Check all servo unit LEDs for other Fanuc servo amplifier alarm codes. If other alarms appear first, clear those before addressing the VRDY OFF.
  9. If the unit is dual-axis: check whether the other axis is undetected/disconnected or has poor contact or locked parameters (e.g., for 0-series system: parameter 8X09#0; for 16/18/0I series: parameter 2009#0).
  10. Check S1, S2 setting on the amplifier: typically S1 = TYPE A, S2 = TYPE B.
  11. If all above are normal → fault may be in CN1 (command line) or the CNC axis control board.

Closing summary & best practices

Always record the exact Fanuc servo amplifier alarm codes shown on the LED before resetting the machine.

Use the manufacturer’s documentation (for example, series A06B-6089/6090) to confirm code meanings.

Follow a logical sequence: power input / supply → cooling/ventilation → wiring/connectors → motor/power-cable → amplifier internals / IPM → parameters/settings.

Keep spare interface boards and IPM modules for quick swap-out in production.

Maintain cleanliness (dust, oil, coolant mist) around servo amplifiers to prevent fan/cooling failures.

Document the incidents: axis name, alarm code, time, machine load condition. This builds a fault history helpful for future preventive maintenance.

For more real repair cases and technical insights, check out our YouTube channel
— we regularly share FANUC servo amplifier alarm troubleshooting and repair videos.

FAQ

1. What does “1 ALM” mean on a Fanuc servo amplifier?

“1 ALM” indicates a fan or cooling alarm. It usually means the internal cooling fan is not working properly due to overheating, dust, or failure. Check whether the fan is running and clean or replace it if necessary.

This alarm occurs when the DC voltage (around 300 V) is below normal. Check the three-phase AC input voltage, the MCC contactor condition, and the detection resistor on the main control board. If all are normal, replace the servo unit.

It means the control power inside the servo unit is too low or the detection circuit has failed. Verify AC input voltage, check the MCC contactor, and inspect the detection resistor. Replace the power or servo unit if needed.

These codes represent abnormal current alarms, often due to a short circuit or IPM (Intelligent Power Module) fault. The letter or number corresponds to the axis involved (e.g., 8 = L axis, 9 = M axis). Check the IPM module, control boards, and motor wiring.

A dotted alarm (e.g., “8.”) indicates an IPM-detected fault such as overheating or an internal short circuit inside the intelligent power module. These issues may not be visible with a multimeter. Replace the IPM or small interface board.

It means the system failed to receive the DRDY (Drive Ready) signal from the servo amplifier. Check wiring, 24 V power, ESP (E-Stop) and MCC contactor circuits, and the LED behavior on the power unit and amplifier.

No. Resetting only clears the alarm temporarily. If the root cause (e.g., power drop, short circuit, IPM fault) remains, the alarm will reappear when the system is restarted.

Ensure good ventilation, clean fans regularly, check AC supply stability, verify grounding, and log all alarm events. Preventive maintenance can significantly reduce Fanuc servo amplifier failures.

You can view the full Fanuc α Servo Amplifier Alarm Code Quick Reference Table on this page, or download the complete Fanuc alarm list from our Downloads page.

If replacing the IPM module, interface board, or checking the wiring doesn’t solve the issue, contact a Fanuc repair specialist or reach out to CNCFixtech for professional diagnosis and repair.

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The customer sends or delivers the servo drive to our company. We register it in our system and prepare it for further inspection.

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