FANUC Alarm 401 — Servo Alarm: VRDY Off
What This Fault Means
Alarm 401 (SV0401) indicates the servo amplifier's VRDY (Voltage Ready) signal is off. The amplifier is not ready to drive the motor, typically due to a power supply or hardware issue in the servo system.
Common Causes
- Servo amplifier internal fault (LED alarm on the amplifier unit)
- DC bus power supply failure
- Emergency stop circuit not fully released
- Contactor for servo power not closing
- Servo amplifier fuse blown
Recommended Fix
- Check the servo amplifier LED display for an alarm code — refer to the amplifier maintenance manual.
- Verify the emergency stop circuit is fully released and the servo-on relay is energized.
- Check the magnetic contactor for servo power — it should close when the machine is enabled.
- Measure the DC bus voltage at the power supply module — should be ~300VDC for 200V class.
- If a specific axis amplifier has a blown fuse, replace and investigate the root cause.
Still stuck? Ask Mira.
Paste your fault code into Mira and get an answer from your actual equipment manuals in seconds.
Try free