Technician operating FANUC CNC testing benches during system inspection and repair at REACO CNC.

FANUC Alarm Code Guide

Find solutions to common Fanuc CNC alarm codes quickly and efficiently.

Servo Alarm
Spindle Alarm
System Alarm
Overtravel Alarm

FANUC Alarms: Quick Guide to Get Back Online​

When your FANUC CNC triggers an alarm, it's trying to tell you something. From simple overtravel to complex servo issues, these codes pinpoint where the problem lies.Our guide cuts through the complexity. We explain the most common alarms—what they mean, why they happen, and how to fix them. Whether you're facing alarm 401 or a stuck overtravel limit, you'll find clear, step-by-step solutions to minimize downtime.​​ You will learn how to:​​

  • Decode any FANUC alarm number.
  • Diagnose faults in servos, spindles, or system memory.
  • Perform basic checks before calling a technician.
  • Identify components that may need adjustment or replacement.

Get the knowledge to solve problems faster and keep your machines running.
FANUC SV0421 Alarm: Excess Error in Semi-Closed Loop Control
The FANUC SV0421 alarm indicates an excess error in semi-closed loop control. This alarm occurs when the difference between the full-closed loop feedback data and the semi-closed loop calculated data exceeds the value set in parameter P2118. In a servo system using both semi-closed loop and full-closed loop feedback, the CNC system compares the motor-side calculated position with the machine-side feedback position. If the difference between these two values becomes larger than the allowable range, the system may generate the SV0421 alarm. This alarm is usually related to incorrect parameter... Read more...
FANUC SV0420 Alarm: Excessive Synchronous Torque Difference
The FANUC SV0420 alarm indicates an excessive synchronous torque difference. This alarm occurs during synchronous operation in feed axis synchronous control when the torque difference between the master axis and the slave axis exceeds the value set in the CNC system. In a feed axis synchronous control system, the master axis and slave axis must operate together with a balanced torque relationship. If the torque difference becomes too large, the CNC system may generate the SV0420 alarm to protect the mechanical structure and servo system. This alarm occurs on the... Read more...
FANUC SV0417 Alarm: Illegal Digital Servo Parameter
The FANUC SV0417 alarm indicates an illegal digital servo parameter. This alarm occurs when an abnormal or invalid parameter setting is detected in the digital servo system. Digital servo parameters are essential for matching the CNC control system with the servo motor, feedback system, rotation direction, and axis control configuration. If one or more digital servo parameters are set incorrectly, the CNC system may detect the abnormal setting and generate the SV0417 alarm. For this alarm, the first important diagnostic point is diagnosis N203#4. The troubleshooting direction depends on whether... Read more...
FANUC SV0415 Alarm: Excessive Movement
The FANUC SV0415 alarm indicates an excessive movement condition. This alarm occurs when a command speed higher than the allowed movement speed limit is specified. In normal CNC operation, the commanded movement speed must remain within the allowable range of the servo control system and machine configuration. If the specified command speed exceeds the movement speed limit, the CNC system may generate the SV0415 alarm to prevent abnormal servo operation. This alarm is mainly related to excessive command speed. In some special applications, such as gear machining using FSC +... Read more...
FANUC SV0413 Alarm: Axis LSI Overflow
The FANUC SV0413 alarm indicates an axis LSI overflow. This alarm occurs when the position deviation counter overflows and the value exceeds the register range. In normal operation, the command value and the feedback value should be almost the same. The value in the error register is generated by subtracting the feedback value from the command value, so the position deviation normally remains close to zero. However, if the command value and feedback value are in opposite directions, or if only one side continues to change while the other side... Read more...
FANUC SV0411 Alarm: Excess Error During Servo Axis Movement
The FANUC SV0411 alarm indicates an excess error during servo axis movement. This alarm occurs when the position deviation of a servo axis becomes too large while the axis is moving. During normal servo operation, the CNC system continuously compares the commanded position with the actual position detected by the feedback system. If the difference between the command and feedback becomes greater than the allowable level during movement, the CNC system may generate the SV0411 alarm. This alarm may be related to incorrect power cable or feedback cable connection, incorrect... Read more...
FANUC SV0410 Alarm: Excess Error at Servo Axis Stop
The FANUC SV0410 alarm indicates an excess error at servo axis stop. This alarm occurs when the position deviation of a servo axis becomes too large while the CNC system judges that the axis should already be in a stopped state. In normal operation, when a servo axis stops, the actual position detected by the feedback system should remain within the allowable position deviation range. If the deviation exceeds the alarm level set in the related parameters, the CNC system may generate the SV0410 alarm. This alarm may be related... Read more...
FANUC SV0409 Alarm: Checked Torque Abnormal
The FANUC SV0409 alarm indicates a checked torque abnormal condition. This alarm occurs when the CNC system detects an abnormal load after the abnormal torque load detection function has been enabled. In simple terms, the control system has detected that the load on a servo motor, Cs axis, or spindle positioning axis is outside the expected range. This may be caused by an actual mechanical overload, incorrect servo-related settings, abnormal machining conditions, or a fault in the servo motor or servo drive. What Does FANUC SV0409 Alarm Mean? SV0409: Checked... Read more...
FANUC SV404 Alarm: Servo Ready Signal VRDY Stays ON
The FANUC SV404 alarm is related to the servo ready signal of the servo amplifier. This alarm occurs when the servo amplifier's VRDY signal, also known as the servo ready signal, remains ON continuously when the CNC system detects an abnormal servo ready status. In normal operation, the CNC system checks the ready status of the servo amplifier before enabling servo control. If the VRDY signal stays ON abnormally, the system may judge that the servo ready circuit, amplifier, or axis control hardware is not operating correctly, and the SV404... Read more...
FANUC SV0403 Alarm: Hardware/Software Mismatch on Servo Axis Card
The FANUC SV0403 alarm usually indicates a hardware/software mismatch in the servo control system. This alarm is commonly related to an incorrect combination between the axis control card and the servo software installed in the CNC system. When this alarm appears, it means the CNC system has detected that the servo-related hardware and software are not correctly matched. In most cases, this is not a simple parameter setting issue. It is usually related to the installed axis card, servo software, or system configuration. What Does FANUC SV0403 Alarm Mean? SV0403:... Read more...
FANUC SV0436 Soft Thermal Relay OVC Alarm: Causes and Troubleshooting Guide
FANUC SV0436, also displayed as SV436 on some CNC systems, is a soft thermal relay alarm related to OVC protection. It means the digital servo software has calculated that the servo motor is operating under excessive load for too long. This alarm is designed to protect the motor and servo system before damage occurs. What Is FANUC SV0436 Alarm? FANUC SV0436 Soft Thermal Relay OVC alarm indicates that the CNC servo software has detected a calculated thermal overload condition. It does not always mean that the servo motor is physically... Read more...
FANUC IE Alarm List: Misoperation Prevention Alarms
This page covers FANUC IE alarms related to the misoperation prevention function, including soft limit detection, excessive commanded rotation speed, illegal acceleration or deceleration, and abnormal machine coordinate position conditions. These alarms are important because they are generated by a preventive protection function designed to detect unsafe or invalid motion conditions before machine damage or incorrect operation can occur. These alarm references apply to FANUC Series 0i-Model D and FANUC Series 0i Mate-Model D. In practical maintenance and machine operation, IE alarms are closely related to motion safety logic, stored... Read more...

Related Resources

Explore more technical materials to help you troubleshoot, maintain, and optimize your CNC system.

Related Links List：

Back to Main Technical Support
Close-up of a CNC machine cutting aluminum parts with high precision.

Need Technical Support or Repairs?

We offer repair services for all CNC parts, including systems, drives, motors, encoders, and PCBs. Fast repair, full testing, and REACO CNC warranty included. Contact us for guidance, troubleshooting, parameter support or repair.

 Tech Support