FANUC Alarm 401 VRDY Off – Causes and Repair Guide

Overview

FANUC Alarm 401 VRDY Off indicates that the servo amplifier is not in a ready state. When this alarm appears, axis movement is disabled and machine operation stops.

This alarm is commonly related to servo amplifier power supply, safety circuits, or hardware failure.

Common Symptoms

* Alarm message: 401 VRDY OFF
* Servo amplifier LED not in ready state
* Axis cannot be enabled
* Machine stops immediately after power on

Main Causes

Servo Amplifier Power Failure

Main power module or internal power supply failure prevents ready signal.

Emergency Stop or Safety Circuit Active

External safety loop not closed or E-stop circuit open.

Blown Fuse or Contactor Issue

Main contactor does not engage properly.

Servo Amplifier Hardware Damage

Internal PCB or IGBT damage.

Diagnostic Procedure

Step 1 – Check Emergency Circuit

Verify E-stop reset.
Confirm safety relays are functioning.

Step 2 – Inspect Servo Amplifier LEDs

Check ready indicator.
Review amplifier diagnostic code.

Step 3 – Verify Power Supply

Measure incoming voltage to servo amplifier.
Check fuses and contactors.

Step 4 – Isolate Servo Unit

If possible, swap amplifier with known working unit for confirmation.

Preventive Maintenance

* Inspect safety circuits regularly
* Keep electrical cabinet clean
* Ensure stable power supply
* Maintain proper cooling airflow

Conclusion

FANUC Alarm 401 VRDY Off is typically related to servo readiness conditions. Proper verification of safety circuits and power modules ensures quick recovery and minimal downtime.