How to Solve Fanuc Spindle Alarm SP9031?

SP9031 is a FANUC serial spindle alarm. It means the spindle motor did not run at the speed the control commanded: it either stopped, or it crept at a very low speed. On the CNC you see SP9031 with SSPA:31 behind it, and the spindle amplifier shows 31 on its own display.

That distinction matters before you order anything. SP9031 is a speed response result, not a part name. A spindle that cannot turn, an open power cable and a failed speed sensor all produce the same alarm.

If SP9031 is on your screen now, send us the CNC alarm text, the number showing on the spindle amplifier, and the amplifier and motor model numbers. We will tell you which of the two diagnostic paths you are on before you order a part.

What SP9031 means on the CNC and on the amplifier

The 0i-D / 0i Mate-D maintenance manual (B-64305EN/03) describes SP9031 as a condition in which the motor cannot rotate at the specified speed. The αi series amplifier maintenance manual (B-65285EN/04) puts the same thing in plainer words: the motor failed to rotate as specified and has stopped or is rotating at a very low speed. Both then split troubleshooting by what the motor actually does. The trigger is the same in both: the speed stays at or below the applicable SST detection level while a rotation command is present.

Reading the same alarm on the spindle amplifier

The CNC and the spindle amplifier report the same event in two vocabularies. The CNC raises SP9031 with SSPA:31 behind it, FANUC's short form for motor lock or disconnected detector. The amplifier shows 31 on its own display, and it uses the same numbering as the rest of the family: SP9002 is 02 on the amplifier, SP9012 is 12, and so on.

Reading both is worth the walk to the cabinet. The CNC message tells you what the control concluded. The amplifier display tells you which module raised it.

Two things get misread here. A plain 31 with no SP prefix in front of it is not a servo alarm, because the SP90xx range is the serial spindle. And the amplifier number does not name the part to replace, it names the module that saw the speed response fail.

One sentence worth keeping: the words "disconnect detector" in the CNC message do not prove the speed sensor has failed. A spindle that is mechanically held, or a motor power circuit that is open, gives the drive the same result: no usable speed feedback.

How to Troubleshoot SP9031 Alarm in Fanuc Machine Systems

When the CNC raises SP9031, the machine side often follows with its own alarm. That second alarm is a consequence, not the cause, so the work starts with SP9031.

FANUC separates the checks by how the motor behaves. Start by watching the spindle when you command a low speed, then take the branch that matches.

If the spindle creeps at a very low speed

• Parameter setting is incorrect. Confirm the sensor related settings against the FANUC AC Spindle Motor Parameter Sheet (B-65280CM).
• Motor phase sequence error. Confirm the motor phase sequence is correct.
• Feedback cable error. Confirm the A and B phase signals are not reversed.
• Feedback cable or sensor fault. Rotate the motor by hand and watch whether a speed appears on the NC diagnostic screen or on the spindle check board. If no speed shows, the cable, the spindle sensor or the motor is what you confirm next.

If the spindle does not turn at all

• The spindle lock sequence is wrong. Confirm the order in which the spindle is locked. On machines with gear ranges, a clamp or a brake, check the shift and clamp signals as well.
• Power cable fault. Confirm the motor power cable is connected correctly and is not open.
• Spindle amplifier module fault. This is the last item on the branch, not the first.

Before either branch, check the load. B-64305EN/03 lists load condition as the first corrective direction for SP9031. Read the load meter at the moment of the alarm, and confirm the spindle, belt, gearbox or coupling can turn freely under your machine builder's procedure. An alarm that only appears under a heavy cut or during acceleration usually belongs here.

The checks that come before you order a part

1. Record the alarm as it happens: the CNC message, the spindle number, the number on the amplifier display, and whether the motor is still, creeping, jerking, or spinning up and then stalling.
2. Note the moment: at start, during cutting, at orientation, or during a gear change. Alarms that appear only in one of those point at different things.
3. Confirm the machine side is not holding the spindle: gear range engaged half way, clamp or brake still applied, tool not released.
4. Check the mechanical side: bearing condition, coupling, belt, and whether the spindle turns by hand with the power off and the main isolator locked.
5. Only then go to the electrical side: motor power cable and phase sequence, then the sensor parameters, then the feedback cable and the sensor itself.
6. Turn the spindle by hand with the power off and read the diagnostic display while you do it. If it will not turn, the problem is mechanical or the spindle is still being held. If it turns and no speed shows, the sensor or the feedback cable is what you confirm next. If it turns and the speed shows correctly, look at the motor power cable for an open phase. On many 0i-D and αi machines the motor sensor arrives at the amplifier on JYA2, so that connector and the cable into it are worth checking before any part is replaced.

Safety: isolate and lock off the supply before you touch the spindle or the amplifier, and let the DC link discharge. Resetting SP9031 repeatedly while the spindle is still held is hard on the motor and the amplifier. If the cause is on the feedback side and the alarm keeps returning, treat it as a stop and fix, especially before rigid tapping or heavy cuts, because a drive that cannot see the real speed can command full torque when you do not expect it.

When the alarm comes back

Three patterns are worth separating, because each one moves the fault to a different place:

• It only appears above a certain speed. Load rising with speed comes first. If your machine has high and low windings or a gear range, the changeover is the next thing to look at: a contactor that does not pull in, or PMC timing that releases the clamp too early, will trip the alarm at the changeover point and nowhere else. The same changeover check comes before the amplifier.
• It only appears in rigid tapping or orientation, while normal turning is fine. Those modes need position feedback. Look at the position coder, its coupling or belt, and its connection, rather than the speed signal.
• It is intermittent. Feedback cable damage is the usual finding: insulation broken down by oil near the motor, or strands broken at the connector. Reseating both ends costs nothing and is worth doing before anything is replaced.

A thread on a public CNC forum from May 2015 shows why the load meter comes first. On a FANUC 0iMC, one particular tool above 1500 rpm drove the spindle load reading to 164 per cent while the tool was not cutting, and tripped 9031. Every other tool ran to 2000 rpm without trouble, on a timing belt drive with no gearbox. The thread closed without a reported fix, so it is not a case with an ending: it is an example of a pattern, and the pattern is that the load meter, not the sensor, is what pointed at the cause.

The order matters for cost as much as for uptime. A feedback cable is a cheap part that takes minutes to confirm. A spindle amplifier is not, and fitting one onto a machine whose real problem is a clamp signal will not clear the alarm.

On machines with a gear range or winding changeover

αi spindle motors widen their output range by switching windings: the high speed winding raises output at high speed, and on the αi P series the low speed winding raises torque (B-65272EN/04). That changeover is both a mechanical and an electrical event, and it is where a good number of speed alarms actually start.

If SP9031 appears at the changeover point and nowhere else, work this order:

1. Confirm the changeover completes. Contactor pulled in, in position switch made, and the PMC sequence in the right order.
2. Look at the contactor and its timing. On machines with high and low windings, the same changeover check comes before the amplifier.
3. Check the spindle lock sequence too. A clamp or brake released a step too early gives the drive the same result: commanded speed, no movement.
4. On machines with gear ranges rather than windings, look at the shift. A range that stops half way leaves the train partly engaged, and the motor sees a load it cannot accelerate.

The clue is the pattern, not the code. If the alarm has a speed threshold and your machine has a changeover, run below and above that threshold before you touch the sensor.

Which speed sensor your spindle uses

The sensor type decides which of the two SP9031 branches you are on, and it is the first thing to re-check after a motor or amplifier swap.

FANUC's αi motor descriptions (B-65272EN/04) put the Mi and MZi choice in terms of the drive ratio: Mi where the spindle already has a sensor and the drive is not 1:1, MZi where the drive is 1:1 and the spindle has none. If a motor was swapped and the sensor type no longer matches the parameter setting, that is branch one of SP9031, the parameter branch, not a failed sensor.

BZi and CZi sit outside the motor and read off a toothed ring, which adds one check the built in types do not need: the gap between the sensing head and the ring. Set it to the value your manual gives for that sensor. One damaged tooth is enough to give speed ripple and an alarm that comes back at particular speeds.

Orientation and rigid tapping need the one rotation signal. When normal turning is fine and only those functions alarm, the one rotation path is where to look, which is why the position coder comes before the speed signal in that case.

Alarms that get mistaken for SP9031

Two of the neighbours in the same SP90xx range (defined in the FANUC serial spindle alarm list, B-64305EN/03) describe problems that feel the same on the shop floor but point somewhere else. Getting the number right before you order is worth the ten seconds.

The pattern behind the last row is worth keeping: on machines with high and low windings, the changeover contactor and the ladder timing around it produce alarms that look electrical but are sequence problems.

If your screen shows a different number, see our guides to the FANUC system alarm 401 and the FANUC servo 430 alarm.

Which part is usually involved

Independent service notes put feedback cable damage at the top of that list, followed by sensor trouble, then the mechanical and machine side causes, with amplifier faults last. Treat it as a starting order only: on your machine it is decided by what you observe, not by a frequency table.

1. Feedback cable and spindle sensor. Checked first because it is cheap to confirm and it is the most common finding. We supply FANUC spindle feedback cables and sensors matched to the motor and amplifier on your machine, including encoder feedback cables such as the A660-2005-T505 and spindle speed sensors such as the A860-2120-T401.
2. Mechanical and machine side causes. Bearings, a gear range that does not complete, a clamp or brake left applied. No part from us fixes these, and we will say so.
3. Parameters and phase sequence. Usually checked after a motor or amplifier has been changed. Confirm the sensor settings against B-65280CM before replacing anything else. On motorised spindles and DD motors, also confirm the power line phase sequence and the relevant sensor or parameter settings for that drive.
4. The spindle amplifier module. The last item, not the first. We supply FANUC spindle amplifiers for CNC, including the αi spindle modules in the A06B-6111 series, and we take the same units in for board level repair.

What the part number tells you before you order

FANUC numbers are built in blocks, and the blocks tell you what you are looking at. A06B-6xxx is the amplifier family: A06B-6111 is the αi spindle amplifier series, A06B-6114 and A06B-6117 are the αi servo modules, and A06B-6110 is the αi power supply. A06B-1xxx is the AC spindle motor series, A660 is the cable family and A860 covers encoders and spindle sensors.

Two practical consequences. First, an αi spindle amplifier is not a servo module with a different label, so a part number from the 6114 or 6117 series will not replace one from 6111. Second, the suffix matters: on the αi spindle modules the same hardware number comes in different voltage classes, and they are not interchangeable. Send the full string off the nameplate, including the part after the hash.

On a typical αi spindle these four parts form one feedback loop, and SP9031 is raised when that loop does not deliver a usable speed signal, so any one of them can be the item you confirm: a spindle motor such as the A06B-1641-B120, a spindle speed sensor such as the A860-2120-T401, a feedback cable such as the A660-2005-T505, and an αi spindle amplifier such as the A06B-6111-H030. FANUC's spindle motor descriptions (B-65292EN/03) list the motor, sensor and cable combinations against their matching amplifier, so the model you send decides which of the others fits.

If you want to see a concrete example, the αi spindle amplifier we most often supply for this alarm is the A06B-6111-H030 spindle module, and the spindle speed sensor in the loop above is the A860-2120-T401. Browse the full range on our FANUC parts page. Parts carry 12 months warranty with optional extended cover on request.

Proactive Measures to Prevent Future SP9031 Alarms

Preventing Alarm SP9031 from occurring in the future is just as important as solving the current problem. By taking proactive measures, you can maintain the efficiency and reliability of your Fanuc system while minimizing downtime. Here are some key steps to consider:

• Regular maintenance and inspections. Schedule regular maintenance inspections to identify and address wear and tear on spindle components such as bearings, belts, and sensors. Perform routine cleaning to remove debris, dust and oil buildup that may be impeding system performance. Check spindle motor alignment to ensure it remains within the manufacturer's specified tolerances.
• Monitor system parameters. Use Fanuc's diagnostic tools to monitor the performance of the spindle motor and associated drives. Watch for abnormal vibration, temperature changes, or power consumption levels, as these can be early indicators of potential problems. Record alarm codes and their resolution to identify recurring problems that may require a more comprehensive solution.
• Upgrade system components. Replace outdated or worn spindle motors, drives, or connectors with newer, more reliable models.

FAQ

What does SP9031 mean on a FANUC control?

SP9031 means the spindle motor did not run at the commanded speed and either stopped or crept at a very low speed. The CNC shows SSPA:31 with it, and the spindle amplifier shows 31. It is a speed response result, so the next step is to establish whether the motor is turning at all.

Does SP9031 mean the spindle sensor has failed?

No. A missing or incorrect speed feedback signal produces SP9031, but so does a mechanically locked spindle and so does an open motor power circuit. Confirm how the motor behaves, and whether a speed appears on the diagnostic display when you turn it by hand, before replacing the sensor, the motor or the amplifier.

Can I keep running by resetting the alarm?

Not for long. If the spindle is mechanically held or a clamp signal is still applied, resetting only reuses the same condition, and it puts extra stress on the motor and the amplifier. If the fault is on the feedback side and the alarm returns, treat it as a stop and fix, particularly before rigid tapping or heavy cuts.

Why does SP9031 only appear above a certain speed?

Because load rises with speed. B-64305EN/03 lists load condition as the first corrective direction for this alarm. On machines with gear ranges, also check that the range change is completing: a shift that does not finish leaves the train partly engaged.

What is alarm 31 on the spindle amplifier display?

It is the same alarm seen from the amplifier side. The CNC reports it as SP9031 with SSPA:31, and the spindle amplifier's own display shows 31. Reading both together is useful, because the amplifier keeps the number even when the CNC screen has moved on.

What information do I need to send for a quote?

The CNC series and version, the spindle amplifier model including the suffix, the spindle motor model, the alarm text with the number on the amplifier display, and when the alarm occurs. The amplifier model alone is enough to start, and we confirm the match against that before quoting.

Can I ask a technical question without buying a part?

Yes. Send the alarm text and the models and we will tell you which of the two paths you are on and what to check first. If the answer is a mechanical or machine side problem rather than a part, we will say so.

Conclusion

Effectively resolving SP9031 alarms and preventing them from recurring is critical to keeping your CNC productive. The order is what saves the money: watch what the motor does, confirm the machine is not holding the spindle, then work through the feedback path, then look at the amplifier.

Send us the CNC alarm text, the number on the spindle amplifier, and the amplifier and motor model numbers. Your engineers stay on site, we work from what you send, and we confirm which part is actually involved before anything ships. If it turns out not to be a part, we tell you that instead.

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Written by the SongWei Robotics technical content team. Last updated October 2026. Reviewed against FANUC maintenance documentation and independent service references listed below.

Safety and scope

Spindle and amplifier work involves mains voltage and stored energy in the DC link. Isolate, lock off and confirm discharge before opening any cabinet or turning a spindle by hand, and follow your machine builder's procedure. SongWei Robotics is not affiliated with FANUC. FANUC and its product names are trademarks of their owner. Where a procedure depends on your machine or control generation, the applicable FANUC manual, your machine builder's documentation and FANUC America's CNC tech tips take precedence.