FANUC SP9072 Alarm 72: Motor Speed Check Result Mismatch and Troubleshooting

FANUC SP9072 means that the spindle amplifier and CNC produced different results when independently checking spindle motor speed as part of Dual Check Safety. The CNC displays SP9072 or SSPA:72 MISMATCH RESULT OF MOTOR SPEED CHECK, while the spindle amplifier displays alarm 72.

This is a redundant safety-check result mismatch. It does not simply mean that the spindle was running too fast, and it does not identify the spindle motor or speed sensor as the failed component. FANUC's official procedures direct technicians to replace the applicable spindle amplifier control printed-circuit board or spindle amplifier, and the applicable CNC CPU, main, sub-CPU, additional spindle, or spindle-interface board, depending on the CNC and amplifier generation.

For surrounding spindle alarms, see the FANUC Spindle Alarm Codes SP9034–SP9092 guide.

What Does FANUC SP9072 Mean?

Dual Check Safety uses independent processing paths so that a single control fault does not silently invalidate a safety function. For spindle speed monitoring, both the CNC side and spindle-amplifier side perform a motor-speed safety check. SP9072 is issued when those two sides do not reach the same result.

The alarm therefore identifies a disagreement between safety-check results, not necessarily a disagreement between two displayed rpm values. FANUC manuals describe it as one of the following equivalent conditions:

• The result of the spindle amplifier speed check does not match the result of the CNC speed check.

• A motor-speed judgment mismatch occurred between the CNC and spindle amplifier.

• The safety-speed check result of the spindle amplifier differs from the CNC result.

The alarm can be issued only when Dual Check Safety is in use on the systems covered by the cited manuals. Because either processing side may be responsible, the alarm number alone cannot determine whether the CNC hardware or spindle-amplifier hardware is defective.

SP9072 Is Not the Same as Safety Speed Exceeded

SP9072 is often confused with SP9069 because both alarms involve motor-speed safety monitoring. Their detection conditions are different:

• SP9069 / alarm 69: the active safety check detected that motor speed exceeded the selected safety-speed limit, or the applicable system detected an abnormality during the resulting stop.

• SP9072 / alarm 72: the CNC and spindle amplifier did not agree on the result of their motor-speed check.

An actual overspeed can correctly produce the same unsafe result in both channels and generate SP9069. SP9072 occurs when the redundant channels disagree, even if the operator cannot see an obvious spindle-speed problem.

Do not attempt to repair SP9072 by raising a safety-speed limit. FANUC's alarm 72 procedure does not specify changing a safety-speed parameter, speed command, gear ratio, or guard-door sequence.

Official Replacement Instructions Vary by FANUC Generation

The technician's proposed control-board and CPU-card replacements are consistent with FANUC documentation, but the exact replaceable unit is series-dependent. “CNC main board,” “CPU card,” “sub-CPU board,” and “spindle-interface board” should not be treated as interchangeable names.

This variation matters. Some amplifier series support replacement of a separate control PCB; others are serviced by replacing the complete spindle amplifier. Similarly, spindle processing may reside on the CNC main board, a sub-CPU card, or a separate spindle-interface/additional spindle board.

Always identify the complete CNC model, spindle amplifier model, hardware suffix, and manual edition before ordering a replacement.

1. Confirm SP9072 and Amplifier Alarm 72

Before removing hardware, verify both displays:

• The CNC alarm screen shows SP9072 or SSPA:72 MISMATCH RESULT OF MOTOR SPEED CHECK.

• The spindle amplifier status display shows 72.

• The alarm is associated with the spindle system using Dual Check Safety.

• No different spindle alarm is being interpreted as alarm 72.

Record the alarm screen before switching off the machine. Also record:

• Complete CNC series and model

• CNC system software series and edition

• Exact spindle amplifier or SPM model, including every suffix

• Spindle software information when available

• Whether the machine has one or multiple spindle interfaces

• Dual Check Safety configuration and applicable safety documentation

• When the alarm first appeared

• Whether a CNC board, spindle amplifier, control PCB, software, parameter, or connector was recently changed

• Any additional CNC, DCS, spindle, or system alarms displayed at the same time

These details are needed to determine which FANUC replacement instruction applies. They also help prevent replacement of a compatible-looking but incorrect board.

2. Do Not Add Unrelated Parameter and Motor Checks

FANUC's official alarm 72 procedures do not instruct technicians to:

• Increase the spindle safety-speed parameters

• Reduce the programmed spindle speed

• Reinitialize spindle motor parameters

• Change motor power-lead phase order

• Replace the motor sensor or feedback cable

• Modify the guard-door ladder sequence

• Adjust spindle acceleration or deceleration

• Reload ordinary spindle parameters as the primary remedy

Those checks belong to other alarms or should be performed only when an additional alarm or independent measurement identifies a separate problem.

For example, SP9073 specifically concerns an abnormal or missing motor-sensor signal. SP9069 concerns safety-speed excess. SP9071 concerns an abnormal safety parameter. SP9072 is the mismatch between the two safety-processing results.

Power cycling can be used only as part of the machine builder's normal confirmation procedure. It is not the hardware repair specified by FANUC, and an alarm that temporarily clears after a reset should not be treated as resolved.

3. Identify the Exact CNC-Side Replacement Unit

Do not order a generic “CNC motherboard” until the installed control architecture has been confirmed. Depending on the FANUC series and spindle configuration, the manual may identify:

• CNC CPU card

• CNC main board

• CNC sub-CPU board

• Additional spindle board

• Spindle-interface printed-circuit board

Use the CNC hardware configuration screen, maintenance manual, electrical drawings, and the complete part numbers on the installed boards. Record all suffixes and option modules.

Before CNC hardware replacement:

• Make a verified backup of CNC parameters, PMC ladder and data, offsets, macro variables, programs, pitch-error compensation, spindle data, and other machine-builder files.

• Preserve the approved Dual Check Safety parameters and required safety signatures or check values using the applicable procedure.

• Record installed options, software editions, board-slot positions, connector locations, and rotary-switch or hardware settings.

• Confirm whether SRAM, FROM, memory, CPU, or option modules must be transferred according to the exact maintenance manual.

• Confirm whether replacement requires software loading, axis or spindle assignment, parameter restoration, or DCS acceptance testing.

A CNC board replacement is not always plug-and-play. Installing an incompatible main board or CPU card can create boot alarms, option mismatches, lost machine data, spindle assignment errors, or additional DCS alarms.

4. Identify Whether the Amplifier Uses a Replaceable Control PCB

FANUC documentation for older αi and some 0i-D configurations refers to the spindle amplifier control printed-circuit board. Later αi-B documentation may direct replacement of the complete spindle amplifier.

Before selecting the repair unit, verify:

• Full spindle amplifier model and all suffixes

• Amplifier series and hardware generation

• Control PCB part number, if the board is separately serviceable

• Compatibility with the spindle motor and power supply

• Supported spindle software and CNC series

• Dual Check Safety support

• Connector arrangement and machine-specific options

Do not assume that every control PCB fitted to an amplifier of the same output rating is compatible. Board hardware, firmware, ID data, option support, and spindle software can differ.

If the applicable manual specifies complete amplifier replacement, do not substitute a control-board-only repair without confirming that the board is an approved service part for that exact unit. If a replacement is needed, browse FANUC servo, spindle, and power drives or send the complete model and suffix to REACO CNC for verification.

5. Follow the Replacement Order for the Applicable Manual

There is no single replacement order that applies to every FANUC generation.

For a Series 0i-D configuration covered by B-64303EN-4/01, the alarm list places the spindle amplifier control PCB first and the CNC main or sub-CPU board second. On an older αi system covered by B-65285EN/04, FANUC states that the CNC CPU card or spindle amplifier control PCB is to be replaced without assigning a universal first component. On αi-B hardware covered by B-65515EN/01, the replaceable units are the CNC CPU card or complete spindle amplifier.

When the exact manual permits either side as the first diagnostic substitution, consider documented evidence such as:

• Whether the alarm started immediately after one unit was replaced or repaired

• Whether a known-good, exact-compatible unit is available for controlled substitution

• Whether another alarm implicates the CNC side or spindle-amplifier side

• Which unit can be replaced without risking loss of machine data or introducing an unverified safety configuration

• Machine-builder service instructions for that model

Component substitution must use exact-compatible, tested hardware and qualified personnel. Do not swap an unknown board from another machine solely because the connector layout appears similar.

6. Replace the Spindle-Amplifier-Side Hardware

If the applicable procedure starts with the spindle amplifier side:

1. Isolate machine power and observe the required DC-link discharge time.

2. Record every connector, setting, board number, and suffix before removal.

3. Install the exact approved control PCB or complete spindle amplifier specified for that series.

4. Confirm all connectors are fully seated and no pins are bent or displaced.

5. Restore only the configuration required by the applicable FANUC and machine-builder procedure.

6. Power up using the approved commissioning sequence.

7. Confirm whether SP9072 clears or whether a different alarm identifies a separate compatibility or configuration issue.

If alarm 72 remains with a verified, compatible spindle-amplifier-side replacement, proceed to the CNC-side unit specified by the exact manual. Do not continue replacing the motor, sensor, or power cables without evidence from another alarm.

7. Replace the Specified CNC Board or CPU Card

If the spindle-amplifier-side replacement does not resolve the alarm, or if the applicable manual directs the CNC side first, replace the identified CPU card, main board, sub-CPU board, additional spindle board, or spindle-interface board.

Before powering up:

• Confirm the full replacement part number and suffix.

• Confirm that required memory and option modules are correctly installed.

• Check board seating, slot position, and connector identification.

• Restore the verified machine backup only through the appropriate FANUC procedure.

• Confirm spindle assignment and DCS configuration.

• Do not copy safety parameters from a different machine.

After replacement, a qualified technician must confirm that the CNC and spindle amplifier are running the intended software and that both DCS channels recognize the same approved configuration.

8. Retest the Spindle and Dual Check Safety

After the hardware repair, perform the machine builder's commissioning and safety-validation procedure. Confirm:

• SP9072 and amplifier alarm 72 no longer appear.

• The spindle amplifier starts without an ID, software, axis-assignment, or safety-parameter alarm.

• Commanded and displayed spindle behavior is normal.

• CNC and spindle-amplifier speed-check results agree throughout the required tests.

• Guard monitoring, safety-speed selection, stop behavior, and MCC-off functions operate correctly.

• Required Dual Check Safety signatures, checksums, acceptance tests, and service records are complete.

The absence of SP9072 after power-up is not, by itself, a complete DCS acceptance test. Replacing either safety-processing side requires the verification specified by FANUC and the machine builder before the machine returns to production.

Difference Between SP9069, SP9071, SP9072, and SP9073

Difference Between SP9072, SP9077, and SP9078

These three alarms are all redundant-result mismatches, but they compare different information:

Do not diagnose SP9072 as a spindle-number setting mismatch or safety-parameter checksum mismatch. Follow the exact displayed number.

FANUC SP9072 Diagnostic Table

What Not to Do With SP9072

• Do not raise safety-speed limits or disable Dual Check Safety.

• Do not change the speed command, motor parameters, or gear ratio without evidence from another fault.

• Do not replace the motor sensor or feedback cable solely because the alarm title contains “motor speed.”

• Do not assume “CPU card,” “main board,” and “sub-CPU board” refer to the same physical part.

• Do not install a spindle amplifier or control PCB selected only by output rating or appearance.

• Do not replace a CNC board without first preserving the complete machine and safety configuration.

• Do not return the machine to service without the required DCS acceptance test.

Recommended Troubleshooting Order

1. Confirm SP9072 on the CNC and 72 on the spindle amplifier.

2. Record the CNC, software, spindle amplifier, control PCB, motor, and DCS configuration.

3. Identify the exact FANUC maintenance and DCS manual for the installed generation.

4. Determine whether the manual specifies a separate spindle amplifier control PCB or complete amplifier replacement.

5. Determine whether the CNC-side service unit is a CPU card, main board, sub-CPU board, additional spindle board, or spindle-interface board.

6. Make and verify all CNC, PMC, spindle, and safety-data backups before hardware work.

7. Replace the first unit specified by the applicable manual using exact-compatible, tested hardware.

8. If the alarm remains, replace the corresponding unit on the other processing side.

9. Diagnose any new alarm by its own number rather than continuing to treat it as SP9072.

10. Complete spindle commissioning and the required Dual Check Safety validation before returning the machine to service.

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 72

• FANUC Series 0i-D Dual Check Safety Connection Manual, B-64303EN-4/01, SP9072 Alarm List

• FANUC AC Servo Motor αi-B Series, AC Spindle Motor αi-B Series, Servo Amplifier αi-B Series Maintenance Manual, B-65515EN/01, Alarm Code 72

• FANUC Series 500i-A Maintenance Manual, B-64805EN/03, SP9072 Alarm List

• Applicable FANUC Dual Check Safety manual for the installed CNC series

Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, DCS option, machine-builder safety circuit, and machine configuration.

Need Help With FANUC SP9072 Alarm?

REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, spindle-amplifier control PCBs, CNC CPU cards, main boards, and spindle-interface hardware. Send us the CNC alarm screen, spindle amplifier display, complete CNC model, exact spindle amplifier model and suffix, control PCB part number, CNC board part number, software edition, DCS configuration, and details of any recent hardware work.

Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The applicable replacement unit and verification procedure depend on the exact CNC, spindle amplifier, software, DCS configuration, and machine.