Reading Fanuc Diag 358 when SV0401 won't clear

Short answer: Diagnosis 358 shows how far each axis's servo amplifier got through start-up before SV0401 was raised. Convert the value to binary and read bits 5 to 14 from the bottom up. The first 0 is the stage that failed: E-stop, MCON, converter ready, DB relay, amplifier ready. Read the bits, not the decimal number.

Applies to: FANUC Series 0i-D / 0i Mate-D and Series 30i/31i/32i-B with αi servo amplifiers (the amplifier manual also lists 16i/18i/21i/0i/PMi). Field reports below include a βi amplifier, an older 0-T and an 18i, labelled as such · Manuals used: FANUC Servo Amplifier αi series Maintenance Manual B-65285EN/04; FANUC Series 0i-MODEL D Maintenance Manual B-64305EN/03; FANUC Series 30i/31i/32i-MODEL B Maintenance Manual B-64485EN/01 · Last verified: 24 Oct 2026

What happens when you can't read Diagnosis 358?

A Charmilles Robofil 230 wire EDM with a FANUC 18i shows 401 VRDY OFF on all five axes, X, Y, U, V and Z, and it won't reset. The owner has already done the obvious checks. He posted them on Practical Machinist: continuity through the E-stop loop at CX4 on the power supply is good, 24 V is present, and the main contactor won't pull in through CX3. Bridge CX3 with a wire and the contactor closes, but the alarms stay. Then the line that stops him: "IN THIS CONTROLLER DAIGNOSIS 358 IS ALSO NOT SHOWING !!"

Four months later another member replied with the same problem on a Charmilles 330F. Nobody posted a fix.

That's what this post is about. Diagnosis 358 is the screen that tells you which step of amplifier start-up didn't finish. When it's there, it splits SV0401 into separate faults with separate check lists. When it isn't, you're working the whole list blind.

What is Diagnosis 358?

Diagnosis 358 is "V ready-off information". FANUC provides it to find the cause of SV0401 (V ready off). Both CNC maintenance manuals give the same short entry: convert the value to binary and check bits 5 to 14. As the amplifier is excited, those bits are set to 1 one at a time, starting from bit 5. On a normal start they're all 1. The first 0 from the bottom is the step that couldn't complete (B-64305EN/03 pp. 21–22; B-64485EN/01 p. 24).

The CNC manuals name only five of those bits, and both say to see the αi servo manual for details. That's the FANUC Servo Amplifier αi series Maintenance Manual, B-65285EN. Section 4.2.2 there gives the full bit layout, a check list, and a table of 358 values with causes (B-65285EN/04, Part I, pp. 18–19). The rest of this post works from that section.

If you haven't yet decided whether you're looking at one axis or all of them, start with Fanuc SV0401: all axes vs one axis. This post picks up at the Diagnosis 358 step.

How do you read Diagnosis 358, bit by bit?

Read it per axis, with the alarm still active, and read it from bit 5 upwards. Here's what each bit means, paraphrased from the amplifier manual (B-65285EN/04 pp. 18–19) and the CNC manuals (B-64305EN/03 p. 22; B-64485EN/01 p. 24):

Bit	Name	What the manual says it is	If this is the first 0
5	HRDY	Shown in the bit layout. Neither manual describes it	Start-up stopped before the E-stop stage. FANUC's value table has no entry for it
6	*ESP	Emergency stop signal (CNC manuals: converter E-stop state released)	The amplifier still sees emergency stop
7	MCONS	Bits 7, 8 and 9 together are the MCON signal, passed from CNC to servo amplifier to power supply	The ready command didn't get through
8	MCONA	(MCON, as above)	As above, one step further
9	MCOFF	(MCON, as above)	As above, one step further
10	CRDY	Power supply preparation completed (CNC manuals: converter ready)	The power supply didn't report ready
11	RLY	Relay signal, DB relay energised	The dynamic brake relay step didn't complete
12	INTL	Interlock signal, DB relay de-energised (CNC manuals: DB relay released)	The DB interlock wasn't seen
13	DRDY	Amplifier preparation completed (CNC manuals: amplifier ready, amplifier)	The amplifier hardware didn't report ready
14	SRDY	CNC manuals: amplifier ready (software)	The last step, software ready, didn't complete

The right-hand column is our plain reading of FANUC's signal names. The ordering is FANUC's.

Two practical notes.

• Stop at the first 0. Bits above it can be 1. FANUC's own example for an E-stop fault (417, below) has bit 6 at 0 but bits 7 and 8 at 1. If you scan for "the highest 1" you'll land on the wrong stage.
• One sentence in the amplifier manual reads the other way. Section 4.2.2 says the first bit "that is not 0" marks the cause. The CNC manuals, the 30i-B supplement printed at the back of the same amplifier manual, and FANUC's value table all only work as the first bit that is 0. Use that.

What do FANUC's listed values point to?

FANUC prints eight Diagnosis 358 values with the problem and what to check (B-65285EN/04 p. 19). We converted each one to binary to show which bit is the first 0. The conversion is our arithmetic. The problem and checks are FANUC's, paraphrased.

FANUC value	Binary (bit 15 … bit 0)	First 0 from bit 5	FANUC's problem	FANUC's checks, in order
417	0000 0001 1010 0001	bit 6, *ESP	Still in emergency stop	E-stop input at CX4 on the power supply released? Then the amplifier-to-amplifier communication cable and connection. Then replace the servo amplifier
97	0000 0000 0110 0001	bit 7, MCONS	(none given)	Is the axis detach function set?
225	0000 0000 1110 0001	bit 8, MCONA	(none given)	Replace the servo amplifier
481	0000 0001 1110 0001	bit 9, MCOFF	(none given)	Replace the servo amplifier
993	0000 0011 1110 0001	bit 10, CRDY	Power supply ready (CRDY) not output	Communication cable CXA2A/B between the amplifiers. Input power present. Power to the magnetic contactor's operating coil, and the CX3 connection on the power supply. Then replace the servo amplifier
2017	0000 0111 1110 0001	bit 11, RLY	(none given)	Replace the servo amplifier
4065	0000 1111 1110 0001	bit 12, INTL	Interlock signal not input	With a dynamic brake module (DBM): its connection to the amplifier, the CX1A/B connection between power supply and amplifier, fuse FU2 on the power supply control board, then the amplifier. Without a DBM: replace the amplifier
8161	0001 1111 1110 0001	bit 13, DRDY	(none given)	Replace the servo amplifier

FANUC also says not to plug or unplug any connector with power on (p. 19). The same section adds a general list before you go to the table: the CXA2A/B cable between amplifiers, the E-stop signal at CX4, whether axis detach has blocked MCON, and whether the amplifier's control board is fully seated. Push the faceplate fully home, and replace the control board if that doesn't help. If none of that explains it, FANUC asks you to report the Diagnosis 358 value to them (p. 18).

Why doesn't my value match FANUC's table?

Because the table's values also carry bits outside the 5–14 range. Every value FANUC lists has bit 0 set and bits 1 to 4 clear. The manual doesn't say what those bits mean or whether they're always that way, and only bits 5 to 14 are meant to be read.

Here's a field reading that shows why it matters. In a set of SV0401 case notes on a Chinese CNC forum (details below), a 0i Mate-TC showed 441 on its Z axis. That isn't in FANUC's table. In binary it's 0000 0001 1011 1001: bits 3 and 4 are set, which none of FANUC's values have, but the first 0 from bit 5 is still bit 6, *ESP. It's the same stage as 417. The fix in that case was in the E-stop path too (see the field table).

So don't search the decimal value. Convert it, ignore bits 0–4 and 15, and find the first 0 from bit 5. We also don't quote a "normal" Diagnosis 358 value. Some websites give one, but it isn't in any of the three FANUC manuals we read.

What should you check, in order?

Follow the machine builder's and FANUC's safety instructions before working in the cabinet. FANUC warns that servo amplifiers stay charged after power-off. Check the residual DC link voltage and the charge LED before you touch one (B-64305EN/03 pp. 179–180).

1. Deal with any other alarm first. The 0i-D check list for SV0401 asks whether the power supply or spindle amplifier has its own alarm, and whether control power, the breaker and MCC drive power are on (B-64305EN/03 p. 385). The all axes vs one axis post covers that split.
2. Read Diagnosis 358 on every axis before you reset or power-cycle. Write the axis name next to each value. Expect: the same first-0 bit on every axis if the cause is shared, such as E-stop or the power supply (our inference). A different value on one axis points at that axis.
3. Convert and find the first 0 from bit 5. Use a calculator in programmer mode or write out the powers of two. Ignore bits 0–4 and 15.
4. First 0 at bit 6 (*ESP): is the E-stop input at CX4 on the power supply released and wired correctly? Then check the CXA2A/B cable between amplifiers. Then the amplifier (B-65285EN/04 pp. 18–19).
5. First 0 at bit 7: check whether the axis detach function is set for that axis, because it stops MCON reaching the amplifier (p. 19).
6. First 0 at bit 10 (CRDY): CXA2A/B cable, then input power to the power supply, then power to the MCC coil and the CX3 connection, then the amplifier (p. 19). This is the stage where the 18i owner's checks above sat: MCC not pulling in through CX3.
7. First 0 at bit 12 (INTL): if the machine has a dynamic brake module, check its connection to the amplifier, CX1A/B, and fuse FU2 on the power supply control board, then the amplifier. No DBM: FANUC goes straight to the amplifier (p. 19).
8. First 0 at bits 8, 9, 11 or 13: FANUC's table goes straight to replacing the servo amplifier for these values. Before you order one, do the general checks from the same section: control board seated and faceplate pushed fully home (p. 18).
9. Nothing found: FANUC asks you to report the Diagnosis 358 value to them (p. 18). Give them the per-axis values you wrote down in step 2.

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

	Manual (B-65285EN/04 §4.2.2)	Field reports
First 0 at bit 10, all axes	CRDY not output: CXA2A/B, input power, MCC coil power and CX3, then the amplifier	Five-axis machine, X/Y/Z/A/B (case notes, Chinese forum): every axis read 993. No three-phase at the power supply, MCC not pulled in. Replacing the power supply didn't help, and disconnecting the spindle amplifier and each servo amplifier one at a time didn't either. Signal F0.6 (SA) read 1, which the engineer took to mean the servo ready signal had been sent and the CX3 contact had closed. The fault was outside the drives: a wire that should have been common, number 100, had no continuity between CX3 and an external relay. The I/O unit and terminal wiring had been swapped with a sister machine and put back without markings, and the two machines differed slightly. Connecting the 100 points fixed it. Single report
First 0 at bit 6, one axis	E-stop not released: CX4, communication cable, then the amplifier	0i Mate-TC, βi amplifier (same case notes): after a Z-axis amplifier side board was replaced, SV5136 cleared but SV0401 appeared on Z. Z read 441 (first 0 at bit 6, our conversion). CX30 on the Z amplifier had nothing plugged in, though the drawing showed a jumper plug. Bridging CX30 pins 1 and 3 cleared the alarm. CX30 isn't in the αi manual; this was a βi amplifier. Single report
358 not available	358 analysis is listed for named controls and, in the /03 edition, from servo software 90B0/D(04) on	FANUC 18i wire EDM (Practical Machinist, 2023): all axes, MCC not energising through CX3, 358 not shown. No fix posted. FANUC 0-T with C-series drives (Practical Machinist, 2024): 401 after drive repairs; the poster understood 358 didn't apply to that control. One reply: "DGN starting at 720 are servo system diagnostics." No fix posted
Confidence	Official	Single reports. The forum case notes are translated by us and paraphrased, not quoted

The 993 case is the useful one. Diagnosis 358 put the fault at the power supply stage on every axis, and the engineer still replaced the power supply first. It took a wiring check against the drawing to find a missing common link left over from earlier work. The 358 value narrowed it to a stage. The wiring change was the cause, and that would have been quickest to find if someone had recorded it.

Fault-code entry (summary)

Field	Value
Code	Diagnosis 358 (V ready-off information), used for SV0401
Brand / series	FANUC / Series 0i-D, 30i/31i/32i-B with αi servo amplifiers (amplifier manual also lists 16i/18i/21i/0i/PMi)
What it shows	How far amplifier start-up got. Bits 5–14 turn to 1 in order. The first 0 is the failed step
Bits	5 HRDY · 6 *ESP · 7–9 MCON (MCONS, MCONA, MCOFF) · 10 CRDY · 11 RLY · 12 INTL · 13 DRDY · 14 SRDY
FANUC values listed	417 (E-stop) · 97 (axis detach) · 993 (CRDY) · 4065 (DB interlock) · 225, 481, 2017, 8161 (amplifier)
Checks in order	1. Other alarms first · 2. Record 358 on every axis before reset · 3. Convert, first 0 from bit 5 · 4. Work FANUC's checks for that bit · 5. Report the value to FANUC if nothing is found
Field fixes reported	All axes 993: missing common wire at CX3 after rewiring (single report). Z axis 441 on a βi amplifier: unplugged CX30 jumper (single report)
Related codes	SV0401 (V READY OFF), SV0404 (V READY ON)
Manual reference	B-65285EN/04 Part I §4.2.2 pp. 18–19; B-64305EN/03 pp. 21–22, 385; B-64485EN/01 p. 24
Last verified	24 Oct 2026

What if Diagnosis 358 isn't on your control?

Then the control or its servo software may predate it. The 2003 edition of the amplifier manual lists 358 analysis for the 16i/18i/21i/0i/PMi, from servo software Series 90B0/D(04) on (B-65285EN/03 p. 25). The 2011 edition adds the 30i/31i/32i (B-65285EN/04 p. 18). Older 0-series controls with C-series drives aren't covered. We can't tell from the 18i thread why 358 wasn't showing on that machine. Servo software edition is one thing to check, but that's our guess, not something the poster confirmed.

Without 358, you're back to the general list: other alarms, E-stop at CX4, MCC and CX3, CXA2A/B, axis detach, amplifier seating. The 720-series diagnostics suggested on Practical Machinist for the older 0-T are a single forum reply, and we haven't checked them against a manual.

Why record Diagnosis 358 before resetting?

Because the value describes the failed start-up, and a reset or power cycle replaces it with a new attempt. If the alarm clears on the second start, the value you didn't write down was the only record of which stage failed. Write it down for every axis, with the date, what was changed recently, and what fixed it. That's the same habit as in "it's running now" is not a fix. The 993 case above came from undocumented rewiring, and a note made on the day of the swap would have pointed straight at it.

How would you find this in your own archive?

If your team has had SV0401 on this machine before, the 358 value and the fix may be in a service report that says "VRDY off", "servo not ready" or "MCC not closing". 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. See how that works for a repeat machine fault, and how we handle your reports.

Book a 20-minute demo: hangersearch.com

FAQ

How do I read Diagnosis 358 on a FANUC control?

Open Diagnosis 358 while SV0401 is active and note the value for each axis. Convert it to binary and look at bits 5 to 14 from the bottom up. They turn to 1 one after another as the amplifier starts, and they're all 1 on a normal start. The first 0 is the start-up step that failed.

What does each bit of Diagnosis 358 mean?

In FANUC's αi amplifier manual, bit 5 is HRDY, bit 6 the emergency stop signal, bits 7 to 9 the MCON signal from CNC to amplifier to power supply, bit 10 power supply ready (CRDY), bit 11 DB relay energised, bit 12 DB relay interlock, bit 13 amplifier ready. The CNC manuals add bit 14, amplifier ready (software).

Diagnosis 358 shows 993 on every axis. What does it mean?

In binary, 993 has its first 0 at bit 10, CRDY, so the power supply didn't report ready. FANUC's checks are the CXA2A/B cable between amplifiers, input power, power to the magnetic contactor coil and the CX3 connection, then the amplifier. In one field report, the cause was a missing common wire at CX3 after rewiring.

My Diagnosis 358 value isn't in FANUC's table. What now?

Convert it to binary anyway. FANUC's listed values also carry bits outside 5 to 14, so a field reading can differ and still point to the same step. A 0i Mate-TC reading of 441, for example, has its first 0 at bit 6, the emergency stop stage, the same as FANUC's 417.

Sources

1. FANUC, AC Servo Motor αis/αi series, AC Spindle Motor αi series, Servo Amplifier αi series Maintenance Manual, B-65285EN/04 (Aug 2011, 276 pp.). Not hosted by FANUC; we read a copy downloaded from mroelectric.com (https://www.mroelectric.com/static/app/product/pdfs/A20B-2101-0390.pdf) on 24 Oct 2026. Supports: §4.2.2 VRDY-OFF check list, full Diagnosis 358 bit layout and signal descriptions (Part I p. 18); Diagnosis 358 value table with problems and checks, and the no-connectors-with-power-on note (p. 19); the 30i-B supplement's "first bit that is 0" wording (supplement B-65285EN/03-02/04, pp. 23–24 of 79, PDF pp. 221–222). Printed page numbers; PDF page = printed + 24 for Part I. Paraphrased.
2. FANUC, same manual, B-65285EN/03 (Mar 2003), copy from resources.industrialautomationco.com (https://resources.industrialautomationco.com/manuals/b-65285en.pdf), read 24 Oct 2026. Supports: Diagnosis 358 supported on 16i/18i/21i/0i/PMi from servo software 90B0/D(04) on (p. 25). This edition has no value table.
3. FANUC, Series 0i-MODEL D / 0i Mate-MODEL D Maintenance Manual, B-64305EN/03. Read as a PDF on 23–24 Oct 2026. Supports: Diagnosis 358 description and bits 6, 10, 12, 13, 14 (pp. 21–22); SV0401 check items (p. 385); residual DC link voltage and charge LED warning (pp. 179–180).
4. FANUC, Series 30i/31i/32i-MODEL B Maintenance Manual, B-64485EN/01. Read as a PDF on 23–24 Oct 2026. Supports: Diagnosis 358 description (p. 24).
5. Practical Machinist, "FANUC 401 ALARM", started by kushan1983, 26 Oct 2023, reply from MEnterprises, 10 Feb 2024: https://www.practicalmachinist.com/forum/threads/fanuc-401-alarm.418158/. Supports: the 18i wire EDM case, the checks done, 358 not showing. No fix posted.
6. Practical Machinist, "Alarm 401 on Fanuc O-T CTX200 machine", started by Igrech72, 11 Sep 2024, reply from Vancbiker: https://www.practicalmachinist.com/forum/threads/alarm-401-on-fanuc-o-t-ctx200-machine.431074/. Supports: 358 not applying to a 0-T with C-series drives; the DGN 720 suggestion. No fix posted.
7. PLC技术网 (plcjs.com) forum, "FANUC | SV0401报警分析及案例" (SV0401 alarm analysis and cases), posted by 数控笔记, 15 Nov 2025: https://bbs.plcjs.com/thread-608569-1-1.html. A compilation of case notes; original authors aren't named. Supports: the all-axes 993 case (missing common wire 100 at CX3 after rewiring) and the 0i Mate-TC Z-axis 441 case (CX30 jumper on a βi amplifier). Translated by us and paraphrased. Single reports.