How to Troubleshoot Fanuc Servo Alarm SV0401

Fanuc alarm SV0401 is the servo ready signal off alarm (VRDY OFF). The control has commanded the axis to run, but the servo amplifier is not reporting that it is ready. The axis stops and the alarm latches until the cause is cleared and the control is reset.

See the full entry in our Fanuc SV0401 alarm code reference. For the related servo alarms SV0410 and SV0417, read Fanuc Servo Alarms: SV0401, SV0410, SV0417 Explained.

What SV0401 Means

For an axis to move, the control expects the servo amplifier to confirm it is ready through the servo interface. If that ready signal is missing or drops while the axis is enabled, the control raises SV0401. It is a readiness problem, not a following-error or overload alarm, so start by asking why the amplifier is not ready.

Common Causes

• Emergency stop or safety circuit open, so the amplifier magnetic contactor never pulls in.
• Amplifier or power supply module fault showing an alarm code or LED on the module.
• Loss of three-phase input power or a tripped breaker or fuse to the servo amplifier.
• Low or missing DC link voltage from the power supply module.
• Damaged or loose servo or FSSB communication cables between the control and the amplifier.
• Incorrect or lost servo parameters after a battery failure, memory clear or software change.

Troubleshooting Steps

1. Read the amplifier first. Look at the status LED or display on the power supply and servo amplifier modules. Another alarm there is usually the real cause.
2. Check emergency stop and safety circuits. Confirm E-stop buttons, door interlocks and the contactor are in the expected state.
3. Verify incoming power. Check the breaker, fuses and all three phases at the amplifier.
4. Inspect cabling. Check FSSB (optical) and signal cables for damage, loose connectors or contamination. Reseat only with power off and locked out.
5. Review recent changes. If the alarm followed a parameter load, battery change or power loss, verify servo parameters against your backup.
6. Reset and retest. After correcting the cause, reset the alarm and jog the axis slowly.

When to Call for Support

If the amplifier shows an internal fault, the DC link will not build, or the alarm returns after the checks above, stop and contact the machine builder or a qualified Fanuc service technician. Servo amplifiers carry hazardous stored voltage; lock out power and wait for the DC link to discharge before working inside the cabinet.

Always follow your machine builder's documentation, because alarm detail and recommended actions vary by control and amplifier series.