FANUC Robot Repair: Does SRVO-021 Prove Amplifier Failure?

FANUC robot repair for SRVO-021 SRDY OFF should begin with the complete alarm record, not an immediate servo amplifier replacement. For the applicable R-30iB Mate and R-30iB Mate Plus controller scope, the alarm means HRDY is ON while SRDY remains OFF, no other alarm explains the state, and the servo amplifier magnetic contactor did not turn on as commanded. Potentially involved systems include controller connections, emergency-stop circuitry, the servo amplifier and power-interruption history. This symptom alone does not prove that the amplifier is damaged.

Symptoms and Applicable Scope

This article addresses SRVO-021 SRDY OFF on FANUC R-30iB Mate and R-30iB Mate Plus controllers covered by maintenance manual B-83525EN/07. It is intended to answer a specific diagnostic question: does this alarm confirm servo amplifier failure? The answer is no. SRVO-021 identifies a servo-ready state that was not achieved under the documented conditions, but diagnosis still requires the alarm sequence, Group and Axis information, controller configuration and inspection results.

Connector locations and procedures depend on the installed controller configuration. Names or layouts from another controller, robot generation or manual revision should not be assumed to apply. Motor, feedback and mechanical inspection should not become the primary diagnostic branch unless an earlier or associated alarm provides evidence for that direction.

Information to Record First

Preserve the evidence before clearing alarms, cycling power or exchanging parts. The earlier event may explain why SRDY remained OFF, while SRVO-021 may only describe the resulting state.

Complete alarm code and full text:
Date and exact time:
Faulted axis:
Robot position/posture:
Program step or motion:
Actual speed and load condition:
Reset result:
Time until recurrence:
Related power, communication or feedback alarms:

For SRVO-021, also record the Group and Axis shown with the alarm. Save the full event log rather than copying only the final message. Identify the robot model, controller model and servo-amplifier label, including the exact model, part number and hardware revision where visible.

Safety and Preparation

Follow the applicable FANUC maintenance information and the factory's lockout, electrical safety and robot safeguarding procedures. Inspection inside a controller can expose personnel to hazardous energy. Only suitably qualified personnel should access energized equipment or observe diagnostic states under controlled conditions.

Do not repeatedly command servo power merely to reproduce the alarm. Repetition without understanding the preceding alarms could increase equipment risk or erase useful evidence. Photograph visible labels, connections and damage before disturbing connectors or replacing components.

FANUC Robot Repair Diagnostic Sequence

1. Record the complete code, Group and Axis. Confirm that the displayed condition is SRVO-021 SRDY OFF and document when it occurred. If any part of the code or affected group is missing, retrieve it from the event log before selecting a repair path.
2. Identify any earlier alarm. Review the chronological alarm history for an event that occurred before SRVO-021. If an earlier alarm explains the servo-off condition, diagnose that alarm first. Do not expand automatically into motor, encoder, feedback or mechanical checks without supporting alarm evidence.
3. Confirm the HRDY and SRDY state. The documented SRVO-021 definition applies when HRDY is ON and SRDY remains OFF, no other alarm explains the state, and the servo amplifier magnetic contactor did not turn on as commanded. Obtain HRDY and SRDY information from the diagnostic screen or maintenance records when qualified personnel can do so safely. If the observed state differs, verify the alarm, controller and manual scope before continuing.
4. Inspect configuration-specific connections. Use the maintenance manual that matches the installed controller configuration and revision. Check relevant connections for secure seating, visible damage, contamination or evidence of prior cabinet work. Do not apply connector names, pinouts or test procedures from a different R-30iB configuration.
5. Check whether an instantaneous power interruption occurred. Compare the alarm time with facility and machine power records. If an instantaneous interruption was recorded, preserve that information and follow the applicable manufacturer procedure. If this information is unavailable, mark it as unknown rather than assuming stable power.
6. Evaluate the emergency-stop unit and servo amplifier. After the preceding checks, qualified personnel may evaluate these assemblies according to the applicable maintenance manual. The decision should be based on verified controller scope, alarm history, connection condition and controlled test findings. SRVO-021 by itself is not confirmation that either assembly must be replaced.

When Repair or Replacement May Be Considered

Professional testing may be appropriate when the evidence continues to point toward the servo amplifier after earlier alarms, power-interruption history and configuration-specific connections have been evaluated. Before sending or purchasing a unit, match the exact model, part number, hardware revision and connector layout. Information about FANUC R-30iB servo amplifier inspection and repair is available at https://autonews.best/products/fanuc-r-30ib-si-fu-qi-ji-xiu.

ZHB is an independent industrial robot inspection, repair and maintenance service provider that also supplies parts to overseas customers; it does not imply FANUC authorization. A repair inquiry should include the controller model, robot model, complete SRVO-021 code, Group and Axis, full event log, alarm time, fault conditions, connector photographs and servo-amplifier label. HRDY and SRDY information may also be supplied when it is available from the diagnostic screen or maintenance records.

Verification After Repair or Replacement

After approved work is completed, verify that all disturbed connections are restored, protective covers are installed and the work area meets the site's restart requirements. Review the alarm history, confirm that the expected servo-ready state can be established, and check for associated alarms. Any controlled motion verification should be performed only by competent personnel under the manufacturer operating information and site safety procedure. Do not treat one successful reset as proof that an intermittent fault has been resolved.

Common Diagnostic Mistakes

Common errors include replacing the servo amplifier from the alarm text alone, ignoring an earlier event, omitting Group and Axis information, using procedures from the wrong controller revision, and moving directly into motor or mechanical diagnosis without an associated alarm. Another mistake is exchanging parts before recording labels and connector photographs, which makes compatibility checks and failure analysis more difficult.

Sources and Update Information

Technical definition and diagnostic priority are based on the FANUC Robot Series R-30iB Mate/R-30iB Mate Plus Controller Maintenance Manual, document B-83525EN/07, owned by FANUC America Corporation. An accessible PDF copy is hosted by Haas Automation at https://www.haascnc.com/content/dam/haascnc/en/service/reference/fanuc-manuals/Fanuc%20Robot%20Series%20R-30iB%20Mate%20+%20Mate%20Plus%20Maintenance%20Manual.pdf. Relevant SRVO-021 information appears on page 73. Updated September 23, 2026.

Conclusion

SRVO-021 SRDY OFF identifies a failed servo-ready transition under defined conditions; it does not independently prove servo amplifier damage. Reliable diagnosis follows the evidence in order: record the full code with Group and Axis, identify earlier alarms, confirm HRDY and SRDY, inspect configuration-specific connections, check for an instantaneous power interruption, and only then evaluate the emergency-stop unit and servo amplifier under the applicable manual.