Post-Holiday CNC Alarm Troubleshooting Guide: Common FANUC Alarms & Solutions

Why Alarms Often Appear After Long Shutdowns

After long holidays or extended machine downtime, CNC machines are more likely to display alarms during restart. The most common causes include battery voltage drop, fan degradation, data loss, and loss of absolute position references.

This article explains common FANUC CNC alarms encountered after restarting machines, their causes, and recommended solutions, helping you restore normal operation quickly and safely.

1. APC Flashing Alarm

Alarm Description

After a long shutdown, the CNC screen may display a flashing APC alarm when the machine is powered on.

Cause

This alarm indicates that the servo amplifier battery voltage is low.

• Normal battery voltage: approximately 6V

• This battery is used to retain the absolute position data of the servo motor

When battery voltage drops below the required level, the CNC can no longer reliably store absolute position information.

Recommended Solution

Replace the servo amplifier battery. After battery replacement, the alarm will be cleared and the machine can resume normal operation.

2. BAT Flashing Alarm

Alarm Description

After long-term machine inactivity, a BAT flashing alarm may appear on the CNC display.

Cause

This alarm indicates low voltage of the CNC system battery.

• Normal battery voltage: approximately 3.3V

• The CNC battery maintains SRAM data, including:

  • CNC parameters

  • PMC parameters

  • Machining programs

If the battery fails completely, critical data may be lost.

Recommended Solution

Immediately replace the CNC system battery.

Please note: If the battery replacement takes too long, the data stored in the SRAM will be lost. Therefore, before performing the replacement, please back up your SRAM data to prevent any data loss.

For detailed backup procedures, please refer to the article: "How to Back Up Parameters."

3. SYS_ALM500 Alarm 

Alarm Description (0i-C: Alarm 935)

After powering on the machine, the following alarm may appear:

(On FANUC 0i-C systems, this alarm may appear as Alarm 935.)

Cause

This alarm indicates that SRAM data has already been lost due to insufficient CNC system battery voltage.

Affected data may include:

• CNC parameters

• PMC parameters

• Machining programs

Recommended Solution

1. Replace the CNC system battery

2. Restore factory parameters or backup data

Parameter restoration should be performed carefully, ideally under professional guidance.

4. FAN Alarm (CNC System Fan)

Alarm Description

A FAN alarm may appear after long-term downtime when the machine is powered on.

Cause

The alarm indicates that the CNC system cooling fan speed is too low or the fan has stopped.

Fan failure may lead to:

• Insufficient heat dissipation

• CNC overheating

• Secondary electronic damage

Recommended Solution

Replace the CNC system fan immediately to prevent further system damage.

5. Servo Amplifier Fan Alarm

Alarm Description

FANUC products contain multiple cooling fans across different modules. Each fan is monitored individually, and alarm numbers vary depending on the component.

Cause

A servo amplifier fan alarm indicates abnormal fan operation in one of the amplifier modules.

Recommended Action

• Refer to the servo amplifier fan alarm number and location reference table (example: FANUC 0i-F system)

• Identify the exact faulty fan or module

Recommended Solution

1. Clean the fan connector and reseat the fan plug

2. Power on and test the system

3. If the alarm persists, replace the faulty fan

6. DS0300 Alarm (Absolute Position Loss)

Alarm Description

After long-term shutdown, the CNC may display a DS0300 alarm.

Cause

This alarm indicates that the machine reference (zero) position has been lost.

Absolute position detection must be re-established before operation.

Recommended Solution

Perform absolute position encoder reference setting, also known as return-to-reference-point operation.

Common homing methods include:

• Mechanical stopper homing

• Marker-based homing

• Non-stopper homing

Because homing procedures vary by machine design, always:

• Refer to the machine operation manual

• Perform the procedure under machine builder guidance if necessary

When to Seek Professional Support

If the alarm cannot be cleared after following the recommended solutions above, professional technical support may be required.

If issues persist, please contact REACO CNC for assistance.

Conclusion

Most post-holiday CNC alarms are related to battery aging, fan degradation, or loss of stored data during extended power-off periods. Performing battery replacement and fan inspection before or immediately after holidays can significantly reduce downtime and prevent data loss.

Proper preventive maintenance ensures a faster, safer return to production.

Data Source: Beijing FANUC

This article is provided for technical reference and CNC maintenance guidance.