FANUC SV0604 means that communication between the servo amplifier and the related amplifier or common power-supply module is abnormal. On the applicable FANUC amplifier, the status display shows P.
For the FANUC Servo Amplifier αi Series covered by Maintenance Manual B-65285EN/04, the official procedure is to check the CXA2A/CXA2B connector and cable, replace the control printed-circuit board if required, and then replace the servo amplifier if the alarm remains. Connector designations can differ on other series; for example, some βi systems use CXA2A/CXA2C.
| Alarm location | Display | Meaning |
|---|---|---|
| CNC | SV0604 / AMP COMMUNICATION ERROR | Communication between the servo amplifier and the associated power-supply/amplifier module failed |
| Amplifier | P | Inter-amplifier communication error |
| αi communication path | CXA2A/CXA2B | Check the applicable connector and inter-amplifier cable |
| Official repair direction | Cable, control PCB, amplifier | Check the communication connection, then replace the control PCB or servo amplifier as necessary |
For nearby servo alarm numbers, see the FANUC SV Alarm Codes: Servo Amplifier, Drive, Voltage and Overheat.
What Does FANUC SV0604 Mean?
The servo amplifier and the common power-supply module operate as a coordinated amplifier group. Besides the main DC power connection, the modules exchange control and status information through an inter-amplifier communication path. SV0604 is generated when the expected communication cannot be established or maintained.
This alarm therefore points first to the communication link and the electronics at either end of that link. It is not described by FANUC as a motor winding, encoder, feedback cable, mechanical load, lubrication, or servo-parameter alarm.
The phrase “communication error between amplifiers” is broader and more accurate than assuming that the common power supply itself has failed. A loose connector, damaged interconnection cable, incompletely seated control board, defective control board, or failed servo amplifier can interrupt the same communication path.
Amplifier Alarm Code P
When SV0604 is present, record both displays before switching off the machine:
- The CNC should show SV0604 or AMP COMMUNICATION ERROR.
- The relevant amplifier should show P.
- Other modules may display a separate alarm or may not become ready.
The CNC alarm number is important because the letter P must be interpreted using the maintenance manual for the installed amplifier series. Do not diagnose the system solely from a single status character without confirming the CNC message and complete amplifier model.
Identify the Installed Amplifier Series Before Checking Connectors
FANUC connector names are not identical across every amplifier generation.
| Amplifier family | Communication connection identified in the applicable manual |
|---|---|
| Servo Amplifier αi Series | CXA2A/CXA2B |
| Certain βi/βiSVSP designs | CXA2A/CXA2C |
Use the connector labels physically marked on the installed modules and the maintenance manual matching the complete FANUC model number. Do not force a connector into a socket merely because another manual shows a similar cable arrangement.
Before ordering a cable, control board, or replacement amplifier, record:
- The complete servo amplifier model and all suffixes
- The complete common power-supply or power-supply-module model
- The CNC series and software information when available
- The connector labels used by the installed amplifier family
- The order of the modules in the amplifier group
- Whether any amplifier, cable, or control board was replaced immediately before the alarm
Common Causes of SV0604
The principal causes are:
- The inter-amplifier communication connector is loose, incompletely inserted, contaminated, damaged, or connected to the wrong socket.
- The CXA2 communication cable is damaged or internally intermittent.
- The servo amplifier control printed-circuit board is not seated correctly or has failed.
- The servo amplifier has an internal hardware fault.
- A recently installed module or control board is not compatible with the amplifier group.
- Another module in the amplifier group has lost control power or has its own primary alarm, preventing normal communication.
The first four items follow the official FANUC fault path most directly. Control-voltage measurement, LED interpretation, and hardware compatibility checks can provide useful supporting evidence, but they must be performed according to the exact amplifier manual and machine electrical drawings.
Safety Before Troubleshooting
The power-supply and servo-amplifier group contains hazardous voltages and can retain energy after incoming power is removed. Inspection or replacement must be performed by qualified personnel.
Before touching the CXA2 connectors, control board, or amplifier:
- Stop the machine using the machine builder's procedure.
- Isolate the incoming electrical supply.
- Apply lockout/tagout as required by the site.
- Wait the discharge time specified for the installed FANUC amplifier.
- Confirm that the DC-link charge indication is off and verify the safe condition using the applicable procedure.
- Do not disconnect or reconnect amplifier communication cables with power applied.
Photograph the existing connector arrangement before removing anything. On a multi-module amplifier group, this record helps prevent connectors from being returned to the wrong positions.
1. Confirm SV0604 and Record Every Amplifier Display
Do not begin by replacing the servo amplifier. First confirm the complete alarm state.
Record:
- The exact CNC alarm number and text
- Which module displays P
- Every character displayed by the common power supply, spindle amplifier, and adjacent servo amplifiers
- Whether the alarm occurs immediately at power-on, after vibration, or after the machine warms up
- Any other CNC, PMC, power-supply, FSSB, or servo alarm shown at the same time
If the common power supply or another amplifier reports a more specific primary alarm, diagnose that alarm as well. SV0604 may be a secondary result when one module in the group cannot power up or communicate normally.
2. Inspect and Reseat the Inter-Amplifier Communication Connectors
After power has been isolated and the discharge procedure completed, inspect the applicable communication path. On the αi systems covered by B-65285EN/04, this is the CXA2A/CXA2B connection.
Check for:
- A connector that is not fully inserted or latched
- Connection to the wrong receptacle
- Bent, recessed, corroded, or contaminated contacts
- Cracked connector housings or broken retaining features
- Oil, coolant, dust, or conductive contamination
- Cable strain caused by an excessively tight bend or unsupported cable weight
- Evidence that the connector becomes intermittent when the cabinet or cable is moved
Reseat the connector only with power safely isolated. Align the connector correctly and never use excessive force. If the connector or cable is physically damaged, replace it with the correct FANUC-compatible part.
3. Inspect the Communication Cable
A cable may appear normal externally while containing a broken conductor or intermittent termination. Give special attention to areas near connectors, cabinet doors, sharp sheet-metal edges, cable clamps, and tight bends.
If the alarm began after cabinet maintenance, amplifier replacement, or cable movement, the communication cable and connector positions deserve priority. A known-good compatible cable can be used for controlled substitution when permitted by the applicable service procedure.
Do not substitute an unverified cable based only on connector appearance. The correct cable must match the installed amplifier family, pin assignment, construction, and required length.
4. Check the Amplifier Group and Control-Power State
The technician-supplied procedure recommends checking the common power supply's 24 V control output and the amplifier LEDs. These can be useful diagnostic checks, but they are not a substitute for the official CXA2 inspection.
Before measuring any voltage, verify the test point, reference point, expected value, and permitted tolerance in the exact FANUC manual and machine electrical diagram. Do not assume that every system exposes or uses the same 24 V point.
Use the module displays to determine whether:
- The common power supply has its own alarm
- An amplifier in the chain has no control power
- One module repeatedly resets
- The modules reach the expected ready state
- The P alarm follows a particular module or cable during an approved substitution test
If another module has a definite power-supply or control-voltage alarm, repair that primary problem before concluding that the servo amplifier is defective.
5. Confirm Compatibility After Recent Hardware Replacement
If SV0604 appeared immediately after a module, control board, or amplifier was replaced, verify compatibility before condemning the new hardware.
Check:
- The complete FANUC ordering number and every suffix
- Amplifier series and hardware revision
- Control printed-circuit board specification
- Compatibility with the common power-supply module
- Connector arrangement and amplifier-group configuration
- Machine-builder restrictions or approved replacement combinations
Physical fit and similar output ratings do not prove compatibility. A module from another series may power up but fail to exchange the expected communication data.
6. Replace the Control Printed-Circuit Board When Applicable
For the αi series procedure, FANUC lists replacement of the control printed-circuit board after the connector and cable have been checked.
This work should be performed only when the installed amplifier design permits field replacement and the technician has the correct board specification and procedure. Before replacement:
- Record the complete amplifier and board numbers.
- Confirm whether machine-specific data or settings must be preserved.
- Observe electrostatic-discharge precautions.
- Ensure that the replacement board is inserted completely and all retaining hooks are engaged.
- Restore each connector to its documented location.
Using an incorrect board or leaving the board partially seated can create additional alarms.
7. Replace or Professionally Test the Servo Amplifier
If the correct cable and connectors are sound, the amplifier group has the proper control-power state, compatibility has been confirmed, and the alarm remains after the applicable control-board procedure, FANUC's next direction is to replace the servo amplifier.
Before selecting a replacement, verify:
- Full FANUC part number and suffixes
- Rated input, output, and axis configuration
- Amplifier family and control-board specification
- Compatibility with the power-supply module, CNC, motors, feedback devices, and software
- Physical connector arrangement
Do not purchase a replacement solely by enclosure appearance or base model number. If a replacement amplifier is required, browse FANUC servo, spindle, and power drives or send the complete models and alarm photographs to REACO CNC for a compatibility check.
8. Retest the Complete Amplifier Group
After the cable, control board, or amplifier has been repaired or replaced, power up the machine according to the applicable FANUC and machine-builder procedure.
Confirm that:
- SV0604 and status code P no longer appear
- The common power supply and all amplifier modules complete startup normally
- No new power-supply, servo, spindle, FSSB, or compatibility alarm appears
- The affected axes become ready and operate under the machine builder's test procedure
- The repaired connection remains stable when the machine reaches normal operating temperature
If a different alarm appears, record and diagnose that new alarm. Do not continue treating every subsequent condition as SV0604.
Difference Between SV0604 and an FSSB Communication Alarm
SV0604 concerns the inter-amplifier communication path, commonly involving CXA2 connectors. An FSSB communication alarm concerns the optical communication path between the CNC and servo amplifier and typically involves COP10 connectors, optical cable, CNC-side communication, or amplifier-side FSSB circuitry.
| Fault type | Main communication path | Typical items checked |
|---|---|---|
| SV0604 | Amplifier-to-amplifier / amplifier-to-power-supply communication | CXA2 connector, inter-amplifier cable, control PCB, servo amplifier |
| FSSB communication alarm | CNC-to-servo-amplifier optical communication | COP10 connector, optical cable, CNC interface, servo amplifier |
Do not replace an FSSB optical cable as the first response to SV0604 unless another alarm specifically identifies the FSSB path.
Difference Between SV0604 and SP9110
SV0604 and SP9110 can both be described generally as communication errors between amplifier modules, but they appear in different alarm systems.
| CNC alarm | Amplifier display | Main context |
|---|---|---|
| SV0604 | P | Servo amplifier communication error |
| SP9110 | b0/B0 | Serial spindle-system communication error between amplifier modules |
Always follow the procedure for the exact CNC alarm and amplifier display. Do not apply the spindle SP9110 procedure automatically to a servo SV0604 alarm.
FANUC SV0604 Diagnostic Table
| Finding | Interpretation | Recommended action |
|---|---|---|
| CNC shows SV0604 and amplifier shows P | Inter-amplifier communication has failed | Record all module displays and inspect the applicable CXA2 connection |
| Alarm appeared immediately after cabinet work | Connector position or cable seating may be incorrect | Isolate power, compare with the documented arrangement, and reseat correctly |
| Alarm changes when the cable is moved | Cable or connector may be intermittent | Replace the correct communication cable and inspect both sockets |
| Common power supply has a separate alarm | SV0604 may be secondary to another module fault | Diagnose the specific power-supply alarm first |
| One module has no display or repeatedly resets | Control power or internal module failure may be present | Check the applicable supply and module procedure |
| Alarm began after amplifier replacement | Compatibility or connector arrangement may be wrong | Verify complete part numbers, suffixes, series, and cable positions |
| Cable and connectors are confirmed good | Control PCB or amplifier hardware may be faulty | Follow the official control-board and amplifier replacement sequence |
| A different alarm appears after repair | A separate fault is now being reported | Diagnose the new alarm number independently |
Recommended Troubleshooting Order
- Confirm SV0604 on the CNC and P on the amplifier.
- Record the displays of every module in the amplifier group.
- Record the complete servo amplifier and common power-supply models and suffixes.
- Isolate machine power and observe the required discharge procedure.
- Check the applicable CXA2 connector positions and seating.
- Inspect the communication cable for damage, contamination, strain, and intermittent connection.
- Diagnose any separate alarm displayed by the common power supply or adjacent amplifier.
- If relevant, check the specified control-power points using the correct manual and electrical diagram.
- If hardware was recently replaced, verify model, board, and revision compatibility.
- Replace the control printed-circuit board when specified and permitted for the installed series.
- Replace or professionally test the servo amplifier if the alarm remains.
- Retest the complete amplifier group and diagnose any new alarm separately.
What Not to Do
- Do not connect or disconnect CXA2 cables while the amplifier group is energized.
- Do not assume that CXA2A/CXA2B applies to every FANUC amplifier series.
- Do not replace motor, encoder, feedback cable, or mechanical components without evidence of a separate fault.
- Do not change servo parameters or PMC logic to bypass a hardware communication alarm.
- Do not assume the common power supply is defective solely because it is one endpoint of the communication path.
- Do not replace the servo amplifier before checking connector seating and the correct inter-amplifier cable.
- Do not install a control board or amplifier based only on physical appearance.
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 P / SV0604
- FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/03, SV0604 alarm list and amplifier troubleshooting
- FANUC AC Servo Motor βi-B/βi Series, AC Spindle Motor βi Series, Servo Amplifier βi-B Series Maintenance Manual, B-65425 series, SV0604 and applicable connector arrangement
Always use the manual edition that applies to the exact CNC, servo amplifier, common power supply, hardware revision, and machine.
Need Help With FANUC SV0604 Alarm?
REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC servo amplifiers, power-supply modules, control boards, and communication cables. Send us the CNC alarm screen, every amplifier display, complete servo amplifier model, complete common power-supply model, connector photographs, and details of when the alarm occurs.
Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.
Reference Source: FANUC technical documentation. The applicable connector designation, replacement procedure, and compatibility requirements depend on the exact amplifier series, hardware configuration, and machine.
