FANUC SP9123 means that the requested spindle-switching state does not match the internal state of the motor/spindle sensor signal switching circuit. The CNC displays SP9123, while the spindle amplifier displays C3.
The affected circuit is the auxiliary module SW, also identified as the submodule SW or SSW. FANUC uses this module in supported spindle-switching configurations where one spindle amplifier controls two spindle motors. The official maintenance procedure identifies the SSW as the primary failed component. On applicable αi-B systems, FANUC also lists the cable between the spindle amplifier and SSW, followed by the spindle amplifier itself.
| Alarm item | Display or component | Meaning |
|---|---|---|
| CNC alarm | SP9123 | Spindle switching circuit error was reported |
| Spindle amplifier display | C3 | The requested and detected sensor-switching states do not agree |
| Switching request | SPSL | Commands the applicable spindle switching state |
| Monitored circuit | Auxiliary module SW / SSW | Switches the motor and spindle sensor signals |
| Main repair direction | SSW, connecting cable, then spindle amplifier | Apply the procedure for the exact amplifier generation |
For nearby spindle and communication alarms, see the Complete FANUC Spindle Alarm Codes SP9110–SP9216.
What Does FANUC SP9123 Mean?
During spindle switching, the CNC or PMC issues the SPSL switching request. The spindle amplifier then checks the internal status of the motor/spindle sensor signal switching circuit in the auxiliary module SW.
SP9123/C3 is generated when these two states do not match:
- The switching state requested by SPSL
- The switching state detected internally in the SW/SSW sensor switching circuit
This alarm concerns the sensor-signal switching path. It does not by itself mean that the main spindle motor, auxiliary spindle motor, motor sensor, spindle sensor, or CNC main board is defective.
What Is the FANUC Auxiliary Module SW or SSW?
The auxiliary module SW supports FANUC's spindle switching function. In a supported machine configuration, one spindle amplifier is switched between two spindle motors, often described as the main-motor side and auxiliary-motor side.
The complete installation can include:
- One FANUC spindle amplifier
- One auxiliary module SW/SSW
- A power-lead switching unit or magnetic-contact switching circuit
- A main-side spindle motor
- An auxiliary-side spindle motor
- Motor and spindle sensor signal connections
- A PMC-controlled spindle-switching request
- Dedicated cables between the spindle amplifier, SSW, switching unit, and sensors
The SSW does not merely route motor power. Its monitored circuit switches the applicable motor/spindle sensor signals so that the spindle amplifier receives feedback from the motor selected by the machine sequence.
How the Spindle Switching System Relates to Alarm C3
When the machine requests a change from one spindle motor to the other, several states must agree:
- The PMC issues the correct switching command.
- The power switching unit changes the applicable motor-power path.
- The SW/SSW changes the motor and spindle sensor signal paths.
- The spindle amplifier confirms the internal sensor-switching state.
- The selected motor and feedback channels agree before spindle operation is enabled.
SP9123 is specifically associated with the mismatch between the SPSL request and the internal sensor-switching status monitored through the SW/SSW. This makes the SSW and its connection to the spindle amplifier the primary diagnostic area.
1. Confirm SP9123 and Amplifier Alarm C3
Before disconnecting hardware, confirm:
- The CNC alarm screen shows SP9123.
- The spindle amplifier status display shows C3.
- The machine is equipped with the FANUC spindle-switching function.
- An auxiliary module SW/SSW is physically installed.
- The alarm occurs during a request to change the selected spindle motor or during startup verification.
- No additional spindle-switching, magnetic-contactor, sensor, or communication alarm is present.
Record:
- Complete CNC series and model
- Complete spindle amplifier model and every suffix
- Complete auxiliary module SW/SSW model and suffix
- Spindle software series and edition when available
- Main-side and auxiliary-side motor models
- All connector and cable positions
- Current SPSL state if it can be observed through an approved diagnostic screen
- Every simultaneous CNC and amplifier alarm
- Whether any cable, SSW, spindle amplifier, contactor, PMC, or motor work was performed beforehand
Photograph the alarm displays and connection layout before switching off the machine.
2. Determine When Alarm C3 Occurs
The timing of the alarm helps identify which part of the switching process requires attention.
| When SP9123 occurs | Checks to prioritize |
|---|---|
| Immediately at power-up | SSW presence, cable connection, wrong module, failed SSW, or damaged spindle-amplifier interface |
| Immediately after SSW replacement | Correct SSW specification, connector positions, cable seating, and installation |
| Immediately after spindle-amplifier replacement | Amplifier compatibility, cable arrangement, and supported switching configuration |
| Only when SPSL changes | SSW switching operation, SSW-to-amplifier cable, and machine switching sequence |
| Intermittently during machine vibration | Loose connector, damaged conductor, poor contact, or failing SSW |
| After PMC or ladder modification | Incorrect switching timing or request state may need review by the machine builder |
| Together with a contactor or power-line switching alarm | Diagnose the related switching-unit or magnetic-contactor condition separately |
Do not assume that the main spindle amplifier has failed merely because C3 is displayed on it. The amplifier is reporting a mismatch detected in the spindle-switching system.
3. Inspect the Cable Between the Spindle Amplifier and SSW
FANUC's αi-B procedure lists the cable connecting the spindle amplifier and SSW as a possible cause. Before replacing a module, isolate machine power, observe the specified DC-link discharge time, and inspect the complete cable set.
Check for:
- A connector that is not fully seated or locked
- A cable connected to the wrong SSW or spindle-amplifier socket
- Bent, recessed, oxidized, contaminated, or heat-damaged contacts
- Broken conductors near connector exits or strain reliefs
- Crushed, stretched, cut, sharply bent, or oil-soaked cable sections
- Loose connector shells or damaged locking hardware
- Incorrect pin assignment in a fabricated replacement cable
- A cable that changes the symptom when moved during an approved test
- Damage introduced when the spindle amplifier, SSW, motor, or switching unit was serviced
The supplied configuration diagram shows several connector groups, including CXA2A/CXA2B and the JYA2/JYA3/JYA4 sensor connections. Their exact use and suffixes depend on the installed SSW and spindle amplifier. Follow the model-specific connection manual rather than moving cables according to appearance alone.
4. Verify the SSW and Spindle-Amplifier Connection Arrangement
Compare the actual installation with the machine electrical drawing and the FANUC manual for the exact auxiliary module.
Confirm:
- The SSW is the correct model for the spindle amplifier and motor arrangement.
- Every spindle-amplifier-to-SSW cable is installed in the correct connector.
- Main-side and auxiliary-side sensor cables are not reversed.
- The cable kit matches the installed spindle amplifier and SSW generation.
- Connector locks, grounding, and shield clamps are intact.
- No unused connector has been mistaken for the active switching connection.
- Recent maintenance did not interchange M-side and S-side sensor connections.
Label each connection before removal. A misconnected feedback path can create additional sensor alarms or unsafe spindle behavior.
5. Check the SPSL Request Without Randomly Editing the PMC
Because SP9123 compares the SPSL request with the SSW's internal switching status, record whether SPSL changes when the machine requests the main or auxiliary spindle.
Review the machine-builder documentation if:
- The alarm started immediately after a PMC or ladder modification.
- SPSL changes at an unexpected point in the switching sequence.
- The command remains active when the machine should be in the opposite spindle state.
- The machine builder recently changed the spindle-switching sequence or interlocks.
However, FANUC's standard C3 procedure identifies the SSW hardware and its cable before the spindle amplifier. Do not edit ladder logic, force SPSL, or bypass an interlock merely to suppress the alarm. Any sequence investigation should be performed by personnel who understand the machine builder's switching design.
6. Replace the Auxiliary Module SW/SSW
FANUC's αi and CNC alarm-list procedures identify a defective SSW as the primary cause of C3. If the connection arrangement is correct and there is no visible cable defect, replace the SSW with a known-good compatible module.
Before replacement, verify:
- Full FANUC order specification
- Every suffix and hardware revision
- Supported spindle-amplifier series
- Supported main-side and auxiliary-side motors and sensors
- Connector arrangement and cable-kit specification
- Machine-builder approved replacement or supersession information
After installing the replacement, restore every cable to its documented position and test the switching sequence before commanding spindle operation.
Do not select an SSW solely by enclosure shape or connector count. A physically similar module may use a different internal circuit or supported configuration.
7. Replace the SSW-to-Spindle-Amplifier Cable if Required
For applicable αi-B systems, FANUC lists the connecting cable as a separate cause after the SSW. If the alarm remains with a confirmed compatible SSW, replace the cable set with the exact specified part.
The replacement must match:
- SSW and spindle-amplifier connector types
- Pin-to-pin assignment
- Main-side and auxiliary-side channel arrangement
- Shielding and grounding construction
- Length and routing requirements
- Machine-specific cable-kit configuration
Change one cable or cable set at a time and retest. This preserves diagnostic value and reduces the risk of reversing several connections simultaneously.
8. Replace the Spindle Amplifier Only After the SSW and Cable
The αi-B maintenance procedure places the spindle amplifier after the SSW and connecting cable. If a correct known-good SSW and correct cable do not clear C3, the spindle amplifier's switching-interface circuit may be defective.
Depending on the exact FANUC service procedure, replace:
- The supported spindle amplifier control printed-circuit board, or
- The complete spindle amplifier module
Before installing a replacement, confirm the full amplifier model, every suffix, spindle software compatibility, connector arrangement, motor support, and SSW support.
If a replacement is required, browse FANUC servo, spindle, and power drives or send REACO CNC the complete amplifier and SSW labels for a compatibility check.
Special Note for βi SVSP Systems
Applicable βi SVSP maintenance documentation states that the βi SVSP does not normally support spindle switching, so C3 is normally not issued in that configuration. If C3 appears on such a system, the manual identifies an abnormal contact involving the switching input at JYA2, including possible contact between pin 17 and another pin such as 5 V.
For a βi SVSP installation:
- Confirm that the machine is not being diagnosed with an αi SSW procedure by mistake.
- Inspect the JYA2 feedback connector and cable for bent pins, contamination, or shorts.
- Use the βi SVSP maintenance manual that applies to the exact unit.
Do not install an SSW simply because C3 is displayed if the installed amplifier family does not support that switching arrangement.
9. Retest the Spindle Switching Function
After the approved repair:
- Restore all cables, connector locks, shields, grounds, and protective covers.
- Confirm that main-side and auxiliary-side connections are not reversed.
- Verify that no tools or loose conductors remain in the cabinet.
- Apply power according to the machine-builder procedure.
- Confirm that C3 and SP9123 no longer appear.
- Observe the SPSL request and switching status through approved diagnostics.
- Test selection of the main-side spindle motor without rotation first.
- Test selection of the auxiliary-side spindle motor without rotation.
- Run each spindle at low speed under controlled conditions.
- Verify speed feedback, orientation, and machine-specific spindle functions for both sides.
- If another alarm appears, diagnose the newly displayed number separately.
Difference Between SP9123, Alarm 15, and Alarm 55
Several alarms can appear on a spindle-switching machine, but they identify different checks.
| CNC alarm | Amplifier display | Main meaning | Primary diagnostic area |
|---|---|---|---|
| SP9015 | 15 | Spindle or output switching sequence error | Switching sequence and magnetic-contactor state |
| SP9055 | 55 | Power-line state does not match the switching request | Switching contactor, power leads, and state-check signals |
| SP9123 | C3 | SPSL does not match the internal sensor-switching circuit state | Auxiliary module SW/SSW and its cable |
Do not replace the SSW for every switching-related alarm. Confirm the exact CNC and amplifier codes first.
Difference Between SP9089 and SP9123
| CNC alarm | Amplifier display | Auxiliary module | Meaning |
|---|---|---|---|
| SP9089 | 89 | SM / SSM | Auxiliary module SM fault associated with a synchronous spindle motor application |
| SP9123 | C3 | SW / SSW | Motor/spindle sensor switching circuit state mismatch in a spindle-switching application |
The SM/SSM and SW/SSW modules serve different functions. Do not order one as a replacement for the other.
FANUC SP9123 Diagnostic Table
| Finding | Likely interpretation | Recommended action |
|---|---|---|
| CNC shows SP9123 and amplifier shows C3 | Sensor-switching state does not match SPSL | Inspect the SSW installation and apply the C3 procedure |
| Alarm started after SSW replacement | Wrong module, loose cable, or incorrect connector position is possible | Verify complete specifications and restore the documented connections |
| Alarm appears only when SPSL changes | SSW switching operation or the SSW-to-amplifier cable is implicated | Check the request state, cable, and SSW |
| Alarm changes when a cable moves | Intermittent conductor or connector contact is likely | Replace the correct cable set |
| Known-good cable does not clear the alarm | SSW is the principal suspect | Replace the compatible SSW |
| Known-good SSW and cable do not clear the alarm | Spindle-amplifier switching interface may be defective | Replace the supported control board or spindle amplifier |
| Alarm started after PMC modification | Switching request timing or logic may have changed | Review the machine-builder sequence without bypassing interlocks |
| βi SVSP displays C3 without an SSW | JYA2 switching-input contact or short may be involved | Inspect JYA2 according to the βi SVSP manual |
| CNC shows SP9055/55 instead | Power-line switching state is abnormal | Diagnose the magnetic contactor and power switching circuit |
| CNC shows SP9089/89 instead | The SM/SSM auxiliary module is involved | Follow the SP9089 procedure rather than the SSW procedure |
Recommended Troubleshooting Order
- Confirm SP9123 on the CNC and C3 on the spindle amplifier.
- Confirm that the machine uses spindle switching and identify the installed SW/SSW.
- Record all module models, suffixes, cable positions, SPSL state, and simultaneous alarms.
- Determine whether the alarm occurs at startup, during SPSL switching, or after maintenance.
- Isolate machine power and observe the required DC-link discharge time.
- Compare the complete spindle amplifier, SSW, sensor, and switching-unit connections with the machine drawing.
- Inspect and reseat the SSW-to-spindle-amplifier cable and all relevant connectors.
- Verify that main-side and auxiliary-side sensor connections are not reversed.
- If recent PMC work was performed, review the documented SPSL sequence.
- Replace the auxiliary module SW/SSW with an exact compatible known-good unit.
- Replace the SSW-to-spindle-amplifier cable set if the alarm remains.
- Replace the supported spindle amplifier control board or complete spindle amplifier only after the SSW and cable are confirmed.
- Retest both spindle selections and diagnose any newly displayed alarm separately.
What Not to Do With SP9123
- Do not replace the spindle amplifier before identifying whether the SSW or cable is faulty.
- Do not treat the SW/SSW as the same module as the SM/SSM used for synchronous spindle applications.
- Do not interchange main-side and auxiliary-side sensor cables.
- Do not move cables according to connector appearance alone.
- Do not force SPSL or bypass the machine's spindle-switching interlocks.
- Do not change PMC ladder logic without the machine-builder documentation.
- Do not disconnect amplifier or SSW cables while power is applied.
- Do not work near the DC link until the specified discharge time has passed and zero voltage has been verified.
- Do not command spindle rotation until the selected motor and feedback paths have been confirmed.
- Do not install an SSW on an amplifier family that does not support that switching configuration.
Technical References
- FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Code C3
- FANUC AC Servo Motor αi-B Series, AC Spindle Motor αi-B Series, Servo Amplifier αi-B Series Maintenance Manual, B-65515EN/01, SP9123 / Alarm Code C3
- FANUC Series 0i/0i Mate-MODEL D Maintenance Manual, B-64305EN/03, Alarm List: SP9123
- Applicable FANUC αi-B Servo Amplifier Descriptions and machine-builder electrical documentation for the auxiliary module SW, cable kit, sensor connections, and spindle-switching sequence
Always use the manual edition that applies to the exact CNC, spindle amplifier, auxiliary module SW/SSW, spindle software, motors, sensors, and machine.
Need Help With FANUC SP9123 Alarm?
REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, spindle control boards, auxiliary modules, switching modules, cables, motors, and sensors. Send us the CNC alarm screen, spindle amplifier display, complete spindle amplifier and SSW models and suffixes, cable and connector photos, machine wiring diagram, and details of when C3 occurs.
Browse FANUC servo, spindle, and power drives, visit the FANUC Technical Support Center, or contact REACO CNC for assistance.
Reference Source: FANUC technical documentation. The supported spindle-switching architecture, connector designations, cable kit, replacement components, and commissioning procedure depend on the installed amplifier generation and machine-builder design.
