# Fanuc SV0401 alarm: all axes vs one axis

SV0401 IMPROPER V_READY OFF means the servo amplifier's ready signal didn't come on. Count the axes before you reset: on every axis, FANUC's check list points at the shared power supply, E-stop, MCC and breaker; on one axis, at that axis. Checks in order, with manual page references.

**Short answer:** SV0401 (IMPROPER V_READY OFF) means a servo amplifier's ready signal, VRDY, didn't come on or dropped out during operation. Before you reset, count the axes. On every axis at once, FANUC's check list points at what they share: power supply, emergency stop, MCC, breaker. On one axis, start with that axis: its first alarm, amplifier and cables.

**Applies to:** FANUC Series 0i-D / 0i Mate-D and Series 30i/31i/32i-B (field reports below also come from older FANUC 0-MC and 10T controls, labelled as such) · **Manuals used:** FANUC Series 0i-MODEL D Maintenance Manual B-64305EN/03; FANUC Series 30i/31i/32i-MODEL B Maintenance Manual B-64485EN/01 · **Last verified:** 23 Oct 2026

## What does it look like when SV0401 shows on every axis?

A 2021 Doosan DNM mill with a FANUC iHMI control boots up one morning with SV0401 on every axis and a spindle alarm next to it. In the owner's words on Practical Machinist: "The control would not reset or EMG Release." He powers the machine off, reboots, and "it has come up fine." There had been a storm overnight, with rain.

He'd already searched, and he'd spotted the gap himself: "What I've found online sounds like a motor ground fault, but those threads don't show the alarm coming up for every axis, and a spindle alarm."

That's the gap this post covers. SV0401 on one axis and SV0401 on all axes are the same alarm number, but they send you to different parts of the cabinet.

## What does SV0401 mean?

SV0401 is "IMPROPER V_READY OFF". The alarm list says the position-control ready signal (PRDY) was on, but the velocity-control ready signal (VRDY) was off (B-64305EN/03, Appendix A, p. 462).

The troubleshooting section adds what that means in practice. The CNC sends MCON to the servo amplifier to ask it to turn the MCC on and activate the servo motor. The amplifier answers with VRDY when the servo is ready. Both travel over the FSSB optical cable. SV0401 is raised when VRDY doesn't turn on, or turns off during operation (B-64305EN/03, section 10.14, pp. 384–385; B-64485EN/01, section 10.14, pp. 561–562).

FANUC lists three places to look:

- **Another servo alarm.** SV0401 can be raised *because* another servo alarm was issued. If so, deal with that first alarm first.
- **The power magnetic circuit around the amplifier.** Power supply, emergency stop, MCC, breaker.
- **The hardware.** The servo amplifier, or the axis cards in the CNC.

## Why does it matter whether it's all axes or one?

Because most of FANUC's check list is shared hardware. The manual's typical connection diagram shows the servo amplifiers and the spindle amplifier side by side on one power supply, with its control power, the emergency stop circuit and the circuit breakers feeding the whole group (p. 385 in B-64305EN/03; p. 562 in B-64485EN/01). Every check item in the power-circuit list (power supply on, E-stop cancelled, MCC on, power to drive the MCC, breaker on, alarm on the power supply or spindle amplifier) is something all those axes depend on.

So the split below is our reading of the manual, not a table FANUC prints:

| What you see | Look first at | First check | Source |
|---|---|---|---|
| **SV0401 on every axis, plus an alarm on the power supply or spindle amplifier** | That other alarm | Deal with the first alarm first. "Has some alarm been issued in the power supply or spindle amplifier?" is one of FANUC's check items | B-64305EN/03 pp. 384–385; B-64485EN/01 pp. 561–562 |
| **SV0401 on every axis, no other alarm** | The shared power magnetic circuit | Power supply control power on? E-stop cancelled? MCC on, including any external MCC sequence? Power for driving the MCC present? Breaker on? On the 30i-B list, also: terminating connector fitted on JX1B of the last amplifier | Same pages |
| **SV0401 on axes that share a multi-axis amplifier, with a servo alarm on one of them** | The axis with the real servo alarm | On a 2-path machine, FANUC says a servo alarm on one axis of a multi-axis amplifier drops that amplifier's MCC and causes SV0401 on the other axes in it. SV0007 is shown alongside to flag this | B-64305EN/03 p. 461 (SV0007) |
| **SV0401 on one axis only** | That axis's amplifier, its cables, the axis card | The other axes are getting power, E-stop and MCC, so the shared circuit is less likely (our inference). Read Diagnosis 358, then work down to amplifier and axis card | B-64305EN/03 pp. 21–22, 385–386; field reports below |

We did the same kind of split for a drive fault in [Siemens MM440 F0001 that won't reset](https://hangersearch.com/blog/siemens-mm440-f0001-fault-wont-reset), where the useful question was *when* it trips. Here it's *how many axes*.

![Ink line drawing of one wide power supply module and a contactor feeding three slim servo amplifiers through a shared busbar, each amplifier cabled to its own servo motor, with the shared power supply, contactor and busbar shaded pale blue](/blog/media/fanuc-sv0401-alarm-all-axes-mvkhf5fz.png)

## What should you check, in order?

Before touching anything inside the cabinet, follow the machine builder's and FANUC's safety instructions. FANUC warns that servo amplifiers stay charged for a while after power-off: measure the residual DC link voltage and check the charge LED is off before touching one (B-64305EN/03, p. 180).

1. **Record before you reset.** Photograph the alarm screen. Write down which axes show SV0401 and every other alarm on screen, including spindle (SP) and power supply alarms. In the Doosan case a reboot cleared everything, and a photo was all that was left.
2. **Find the first alarm.** If another servo alarm is present, FANUC says to act on that one first (p. 384). On a 2-path machine with SV0007, go to the axis in the other path (p. 461).
3. **Read Diagnosis 358 (V ready-off information) while the alarm is active, and note the value for each axis it's shown for.** Convert the value to binary and look at bits 5 to 14. When the amplifier is excited, these bits are set to 1 one after another, starting at bit 5. If start-up is normal, bits 5 to 14 are all 1. The first bit that is still 0 is the step that didn't complete (B-64305EN/03 pp. 21–22; B-64485EN/01 p. 24). The bits the manual names are:
   - bit 6 \*ESP: converter emergency stop state released
   - bit 10 CRDY: converter ready
   - bit 12 INTL: DB relay released
   - bit 13 DRDY: amplifier ready (amplifier)
   - bit 14 SRDY: amplifier ready (software)

   **Expect:** the same first-zero bit on every axis if a shared item is the cause (our inference). **If** only one axis has a zero, stay on that axis.
4. **Every axis: work the shared list.** Power supply control power on. Emergency stop cancelled. MCC on, and if the machine builder added an external MCC sequence besides the power supply's own MCC contact, check that too. Power for driving the MCC present. Breaker on. On a 30i-B, check the terminating connector on JX1B of the last amplifier (B-64485EN/01 p. 562).
5. **Every axis: check the power supply's own 24 V.** In one Practical Machinist thread (details below), an old FANUC 0-MC threw "401 VRDY off" with all three axes not ready, and the MCC contactor never pulled in. The owner measured the 24 V circuits on the power supply module and found them fluctuating, roughly 14–20 V on CX4 and 16–22 V on CX2B. Replacing the power supply module fixed it. That's a single report on an older control.
6. **One axis: check that axis's motor power cable.** On a FANUC 10T lathe with "z improper v ready off", a reply suggested: "If this mostly happens when trying to turn on the machine, I'd carefully check the motor power cabling and connections for evidence of a short. Including removal of the shell on the connector at the motor." Then megger the motor. Another member had found a cable that was "shorted in the cableway" by disconnecting the motor wires at both ends and meggering them. Take IR pass/fail values from the motor maker or the applicable standard, not from a forum.
7. **No fault in the power circuit: the amplifier, then the axis cards.** FANUC's order is to replace the servo amplifier if nothing is wrong in the power magnetic circuit around it, and to replace the axis cards if that doesn't solve it (B-64305EN/03 pp. 385–386; B-64485EN/01 pp. 562–563).

![Ink line drawing of a machine tool ball-screw axis on a cast base with linear guides and a saddle, the servo motor's power connector unplugged with its shell lying beside it, and a cable carrier chain running back from the motor, with the connector and cable carrier shaded pale blue](/blog/media/fanuc-sv0401-alarm-all-axes-mvkheshp.png)

## What does the manual say vs what has fixed it in the field?

| | Manual (B-64305EN/03 section 10.14; B-64485EN/01 section 10.14) | Field reports |
|---|---|---|
| **All axes** | Act on any other servo, power supply or spindle alarm first. Then: power supply control power, E-stop, MCC and external MCC sequence, MCC drive power, breaker. Then the amplifier, then the axis cards | **Doosan DNM, FANUC iHMI (2023):** SV0401 on every axis plus a spindle alarm at boot. Cleared on a power cycle. Replies suggested the MCC (from the FANUC app's reading of the spindle alarm) and the 24 VDC supply. **No confirmed fix.** **FANUC 0-MC knee mill (2026):** 401 with all axes not ready, MCC never energised, unstable 24 V on the power supply module. **Fixed by replacing the power supply module** (confirmed by the poster). Single report, older control |
| **One axis** | Same list, then the amplifier, then the axis cards | **Daewoo Puma 6, FANUC 10T (2018):** Z-axis V-ready off, which the poster linked to cold weather. Replies: check the motor power cable and connector for a short, then megger. One member's own fix on a different 10M machine was a main-board fuse that "tested good, but wasn't making a good connection". **No confirmed fix from the poster** |
| **Confidence** | Official | Single reports, mixed control generations |

Two things are worth taking from those threads. First, the all-axes cases both point at shared hardware (MCC, power supply), which is where FANUC's list starts. Second, neither of the unresolved threads recorded Diagnosis 358 or the alarm history, so there was little to go on when the alarm cleared on its own.

## Fault-code entry (summary)

| Field | Value |
|---|---|
| Code | SV0401 |
| Brand / series | FANUC / Series 0i-D, 30i/31i/32i-B (same alarm message in both manuals) |
| Official name | IMPROPER V_READY OFF (section heading: V READY OFF) |
| Trips when | VRDY doesn't turn on at start-up, or turns off during operation, while PRDY is on |
| Key split | Every axis: shared power circuit first · One axis: that axis's alarm, amplifier, cables |
| Checks in order | 1. Record alarms before reset · 2. Act on the first alarm · 3. Diagnosis 358, first zero bit from bit 5 · 4. Power supply control power, E-stop, MCC, MCC drive power, breaker (30i-B: JX1B terminator) · 5. Servo amplifier · 6. Axis cards |
| Field fixes reported | Power supply module replaced on a 0-MC with all axes down (single report). One-axis suggestions: motor cable short, megger, main-board fuse contact (unconfirmed) |
| Related codes | SV0404 (V READY ON), SV0007 (servo alarm on another path of a multi-axis amplifier) |
| Manual reference | B-64305EN/03 pp. 461–462, pp. 384–386, pp. 21–22; B-64485EN/01 pp. 561–563, p. 24 |
| Last verified | 23 Oct 2026 |

## Why did SV0401 clear after a power cycle?

The manual doesn't say a power cycle fixes SV0401. It says VRDY didn't come on or dropped out. A reboot that comes up clean tells you the condition wasn't there on the second start. It doesn't tell you what it was. On the Doosan thread one reply was simply "see if it repeats", which is fair advice, but only if the first event was written down well enough to recognise the second one.

That's the same problem as any machine that's "running now". What to write before closing the job (alarm list, which axes, Diagnosis 358 value, weather or power events, what was changed) is in our checklist: ["it's running now" is not a fix](https://hangersearch.com/blog/its-running-now-is-not-a-fix). The Puma 6 owner's note that it "happens when it gets cold out" is exactly the kind of detail that makes the next occurrence easy to match.

## Is it the amplifier, the power supply or the cable?

Count the axes, then read Diagnosis 358. If every axis has the alarm and the same first zero bit, start with the shared power supply, E-stop and MCC circuit. FANUC lists those before the amplifier. If one axis has it, the shared circuit is working for the others, so look at that axis's motor cable and connector, then its amplifier, then the axis card. FANUC puts the amplifier before the axis cards.

## How would you find this in your own archive?

If your team has seen SV0401 on this machine before, the fix is probably in a service report written under some other description: "servo not ready", "all axes alarm", "MCC". HangerSearch searches your own finalized service reports by alarm code, machine or symptom. It shows the few prior cases that match and why each one matched, then opens the cited page. Nothing is generated, and if there's no similar report it says so. How that works for a [repeat machine fault](https://hangersearch.com/blog/find-past-service-fix-repeat-machine-fault), and [how we handle your reports](https://hangersearch.com/trust).

Book a 20-minute demo: hangersearch.com

## FAQ

### What does SV0401 IMPROPER V_READY OFF mean on a FANUC control?
The servo amplifier's velocity-control ready signal, VRDY, was off while the position-control ready signal, PRDY, was on. In practice VRDY didn't turn on at start-up or dropped out during operation. FANUC lists three areas: another servo alarm that triggered it, the power magnetic circuit around the amplifier, or a faulty servo amplifier or axis card.

### Why is SV0401 showing on all axes?
All the axes share the power supply, emergency stop circuit, MCC and breaker, so a fault there can take them all down together. Check for an alarm on the power supply or spindle amplifier first, then power supply control power, E-stop, MCC and MCC drive power, and the breaker. In one forum report on an older 0-MC, replacing the power supply module fixed it.

### SV0401 is only on one axis. Where do I start?
Read Diagnosis 358 while the alarm is active and find the first bit, from bit 5 upwards, that is 0. Since the other axes are getting power and MCC, check that axis's motor power cable and connector for a short, then megger the motor. FANUC's order after the power circuit is the servo amplifier, then the axis cards.

### How do I read Diagnosis 358 for SV0401?
Convert the value to binary and look at bits 5 to 14. They are set to 1 in sequence from bit 5 as the amplifier starts, and all are 1 when start-up is normal. The first 0 marks the step that didn't complete: bit 6 E-stop released, bit 10 converter ready, bit 12 DB relay released, bit 13 or 14 amplifier ready.

## Sources

1. FANUC, Series 0i-MODEL D / 0i Mate-MODEL D Maintenance Manual, B-64305EN/03. Not hosted publicly by FANUC; we read a copy as a PDF on 23 Oct 2026. Supports: SV0401 and SV0404 alarm list entries (Appendix A, p. 462) and the SV0007 multi-axis amplifier note (p. 461); section 10.14 cause, MCON/VRDY over FSSB, typical connection diagram and check items (pp. 384–385); replace the amplifier, then the axis cards (pp. 385–386); Diagnosis 358 bits (pp. 21–22); DC link residual voltage warning (p. 180). Paraphrased; page numbers are the printed numbers.
2. FANUC, Series 30i/31i/32i-MODEL B Maintenance Manual, B-64485EN/01. Read as a PDF on 23 Oct 2026. Supports: section 10.14 check items, including the JX1B terminating connector (pp. 561–563); Diagnosis 358 (p. 24). Paraphrased.
3. Practical Machinist, "Fanuc Doosan SV0401 Improper V Ready Off ALL Axes", started by runninwideopen, 10 Jan 2023, replies from dandrummerman21, GENERALDISARRAY and Ox: https://www.practicalmachinist.com/forum/threads/fanuc-doosan-sv0401-improper-v-ready-off-all-axes.409281/. Supports: the all-axes case at boot, the spindle alarm, clearing on a power cycle, and the MCC and 24 VDC suggestions. No confirmed fix.
4. Practical Machinist, "Alarm 400, 401, 4n4 on fanuc 0-MC OM controller (maybe an MCC issue)", started by pasper20, 10 Feb 2026, resolved by the poster 1 Mar 2026: https://www.practicalmachinist.com/forum/threads/alarm-400-401-4n4-on-fanuc-0-mc-om-controller-maybe-an-mcc-issue.446392/. Supports: 401 with all axes not ready, MCC not energised, fluctuating 24 V at CX4 and CX2B, fixed by replacing the power supply module (single report, older control).
5. Practical Machinist, "Fanuc 10t SV013???", started by fuquamatt, 7 Apr 2018, replies from Garwood and Vancbiker: https://www.practicalmachinist.com/forum/threads/fanuc-10t-sv013.348945/. Supports: the one-axis (Z) V-ready off case, the cold-weather observation, the motor cable/connector and megger suggestions, and the main-board fuse report. No confirmed fix from the poster.