Get Started Free

FANUC SV0401 VRDY OFF after power-on or E-stop reset, what to check?

ServoFANUCSV0401
Question

I'm getting a FANUC SV0401 VRDY OFF alarm right after powering on the machine or after resetting an E-stop. The servo doesn't seem to be ready. What should I check first to get the servo ready?

Top answer · Written by InduQT from the manufacturer manuals cited below · Nov 4, 2026

The SV0401 VRDY OFF alarm indicates that the servo amplifier is not ready to operate. This often points to an issue in the amplifier ready chain, the main contactor (MCC), or the emergency stop circuit. You'll need to check the status of these components to identify why the VRDY signal isn't active.

Likely causes and checks

1Emergency Stop (ESP) Circuit

Points to it
The alarm occurs immediately after power-on or E-stop reset, suggesting the E-stop signal might still be active or not properly reset. Diagnosis 358 bit #06 (*ESP) will be 0.
Check
Verify that all E-stop buttons are released and the E-stop circuit is closed. Check the wiring to the CX4 connector on the PSM (Power Supply Module) for continuity and proper connection. If available, check diagnosis information No. 358 and confirm bit #06 is 1.

2Main Contactor (MCC) or Converter Ready (CRDY)

Points to it
If the E-stop is clear, the next step in the ready chain is often the main contactor or the converter. Diagnosis 358 bit #10 (CRDY) will be 0 if the converter isn't ready, or bits #07, #08, #09 (MCON) if the CNC isn't commanding the amplifier.
Check
Inspect the main contactor (MCC) for proper operation and auxiliary contact status. Check for any blown fuses in the power supply to the converter. If available, check diagnosis information No. 358 and confirm bit #10 (CRDY) is 1. Also, check bits #07, #08, #09 (MCON) to ensure the CNC is sending the ready command.

3Amplifier Ready Chain (SRDY, DRDY, INTL, RLY)

Points to it
If the E-stop and converter are good, the issue could be within the amplifier's internal ready sequence. Diagnosis 358 bits #11 (RLY), #12 (INTL), #13 (DRDY), or #14 (SRDY) will be 0, indicating a specific stage of the amplifier's readiness is not met.
Check
With power isolated and locked out, visually inspect the servo amplifier for any obvious damage or tripped internal breakers. Check the communication interface cable (CXA2A/B) between the amplifier and module for correct connection. If available, check diagnosis information No. 358 and sequentially verify bits #11 through #14 are 1.

Next check: If the above checks do not resolve the issue, consult the FANUC maintenance manual for your specific model and check diagnosis information No. 358 (V ready-off information) on the diagnosis screen. Convert the displayed value to binary and check bits 5 to 14 sequentially, starting from the lowest-order bit, to pinpoint the exact stage where the ready signal is failing.

What it typically looks like in the data

VRDY OFF (E-stop active)0 s2 s4 s6 s8 s10 s10*ESP (E-stop) (State)10CRDY (Converter Ready) (State)10VRDY (Servo Ready) (State)
This chart illustrates the sequence of ready signals leading to a VRDY OFF alarm, showing the E-stop signal remaining active. Illustrative, synthetic data showing the typical pattern, not a recording from a real machine; values vary by machine and settings.
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.

Related questions

Different symptoms on your machine?

Describe the fault or upload a photo, and InduQT checks it against the manufacturer manuals. Share the result here when it helps someone else.

Ask the free troubleshooter