FANUC Series 0i-F / 0i-F Plus alarm SV0401: IMPROPER V_READY OFF
SV0401 (IMPROPER V_READY OFF) on a FANUC 0i-F means the velocity-control ready signal (VRDY) from the servo amplifier was OFF although the position-control ready signal (PRDY) was ON. In practice the amplifier stopped being ready to drive the motors, so SV0401 is usually a consequence: the root cause is whatever made the amplifier drop out.
Diagnose this with AI
Try Free CNC Alarm Diagnostic Upload a keypad photo, the fault history or a parameter export from your FANUC Series 0i-F / 0i-F Plus. InduQT compares the evidence and suggests the next useful checks.
When it happens
- At power-up or when the emergency stop is released (amplifier never becomes ready)
- During operation, together with another servo, power-supply or spindle alarm
- On every axis of the same amplifier at once
Likely causes and checks
1Another amplifier or power-supply alarm dropped the MCC
- Points to it
- Other SV or SP alarms appear in the same alarm screen or alarm history at the same time; several axes show SV0401 together.
- Check
- Read the full alarm list and the amplifier displays, and clear the first alarm first. FANUC notes that on a multi-axis amplifier shared between paths, an alarm on one path’s axis drops the MCC and raises SV0401 (with SV0007) on the other path.
2Emergency stop chain or converter not ready
- Points to it
- Alarm appears when releasing E-stop or at power-up; DGN 358 bit *ESP or CRDY is 0.
- Check
- Read DGN 358 as binary and find the first 0 between bits 5 and 14: *ESP (converter E-stop released), CRDY (converter ready), INTL (DB relay released), DRDY and SRDY (amplifier ready). The first missing bit is the step that failed.
3Dynamic-brake relay or amplifier hardware not reaching ready
- Points to it
- DGN 358 shows *ESP and CRDY set but INTL or DRDY missing; repeats on the same amplifier.
- Check
- Note the amplifier LED and the DGN 358 value and compare with the FANUC alpha i servo amplifier maintenance manual before replacing the amplifier.
4Low supply to the power supply module
- Points to it
- Alarm coincides with plant supply dips or other machines starting; may come with SV0433 or SV0435 (low DC link).
- Check
- Have a qualified electrician measure the three-phase input to the power supply module under load and compare with its rating.
Telling it apart
- SV0404 (IMPROPER V_READY ON) is the opposite: VRDY is ON while PRDY is OFF.
- SV0460 (FSSB DISCONNECT) means the optical servo link itself was lost, for example a broken FSSB cable or an amplifier without power.
- If SV0401 is the only alarm, DGN 358 is the fastest way to see which step of the amplifier start-up sequence failed.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| DGN 358 (diagnosis) | V ready-off information | Bits 5–14 are set in sequence during amplifier start-up; the first bit still 0 shows where the sequence stopped. |
Evidence to collect
- All alarms in the alarm screen and alarm history with times
- DGN 358 value for each axis showing SV0401
- Amplifier and power supply display readings
- Whether the alarm appears at power-up, at E-stop release or while running
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