What the SRVO-021 alarm means
The servo amplifier for the specified axis did not report ready when servo power was switched on, so the controller cut servo power. We are very familiar with this alarm on FANUC R-30iA / R-30iB / R-30iB Plus systems — usually the first on-site inspection is enough to confirm the cause.
Most common causes
- Servo amplifier fault for that axis
- E-stop/safety circuit not closed
- Power cable or signal cable between controller and amplifier loose
- Axis control card failure
Step-by-step field fix
- Check the 7-segment LED on the servo amplifier for its own alarm code
- Verify E-stop chain is reset and chain circuit is healthy
- Inspect power and signal connectors on the amplifier
- Swap the amplifier with a known-good unit to isolate amp vs. wiring
How we diagnose servo alarms on site
FANUC srvo-021 belongs to the servo & drive family. On FANUC srvo-021 the failing stage is alpha-i / beta-i servo amplifiers with PSM supply modules and SVM servo modules (SPM where spindles fit) on the R-30iB Plus / R-30iB Mate Plus / R-30iA - the servo closed loop: current/velocity/position loop misbehaviour, motor-drive matching and load identification. We start reading FANUC srvo-021 at the iPendant ALARM screen (SRVO / MOTN / SYST / INTF prefixes) together with the amplifier's 7-segment status code.
The chain involved in FANUC srvo-021 runs: target -> position loop -> velocity loop -> current loop -> power output -> motor -> feedback -> loop compare.
The classic misjudgement on FANUC srvo-021 is here: The easiest misjudgement in this family is blaming the drive for mechanical drag. The discriminator is the current waveform: normal at no load but clearly raised and wavering under load points at mechanics (reducer, bearings, assembly stress); abnormal already at no load, or spiky and oscillating, points at electrics (drive, motor windings, feedback). We capture the waveform on site before concluding anything, and decide parts after measurement - which saves customers paying for swaps they did not need.
Field notes on this page cover FANUC srvo-021 (SRDY Off (Group / Axis)); written by the Henghuan repair team, updated 2026-10-01.
Real Field Case
Machine: FANUC ARC Mate 120iD (2019)
Symptom: A 2019 arc-welding robot raised SRVO-021 (SRDY off) and the servo would not power up.
What we did: We checked the enable chain and amplifier supply, found a blown control-power fuse on one amplifier, replaced it and verified no short before re-energizing.
Result: Servo readied normally and welding resumed in about 2 hours.
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Related Servo & Drive alarms
SRVO-023 — Stop Error Excess (Group / Axis) SRVO-024 — Move Error Excess (Group / Axis) SRVO-043 — DCAL Alarm - Discharge Alarm SRVO-044 — HVAL Alarm - High Voltage SRVO-045 — HCAL Alarm - High Current SRVO-046 — OVC Alarm - Overcurrent (Average)Frequently asked questions
QWhat does the FANUC srvo-021 alarm mean?
The servo amplifier for the specified axis did not report ready when servo power was switched on, so the controller cut servo power. We are very familiar with this alarm on FANUC R-30iA / R-30iB / R-30iB Plus systems — usually the first on-site inspection is enough to confirm the cause. FANUC srvo-021 sits in the servo & drive family; on our work orders the highest-frequency trigger for this code is "Servo amplifier fault for that axis".
QCan I clear a FANUC srvo-021 alarm myself?
Whether FANUC srvo-021 clears on its own depends on the outcome of "Check the 7-segment LED on the servo amplifier for its own alarm code". Finish the flow with "Swap the amplifier with a known-good unit to isolate amp vs. wiring" to confirm FANUC srvo-021 - do not skip it. If that step is clean, the second most frequent cause on FANUC srvo-021 is "E-stop/safety circuit not closed". Do not chase takt through a repeating FANUC srvo-021. Left alone, servo & drive faults tend to graduate from repairable to must-replace.
QFor servo alarms like FANUC srvo-021, how do you tell motor from drive?
The commonest causes we see on FANUC srvo-021 are "Servo amplifier fault for that axis, E-stop/safety circuit not closed, Power cable or signal cable between controller and amplifier loose", so we stock the van against those. For FANUC srvo-021 specifically, we bring the scope and meter; motors and drives are stocked by type, and most cases are measured before parts are ordered. For FANUC srvo-021, on servo alarms we specifically avoid misjudgement: waveform first, parts second, so customers skip needless swaps. Before we roll, tell us what FANUC srvo-021 shows on the iPendant ALARM screen (SRVO / MOTN / SYST / INTF prefixes) together with the amplifier's 7-segment status code and what the machine was doing - phone or video triage is free.
The content on FANUC srvo-021 was written by Henghuan Robotics service engineers from hands-on repair experience. Alarm descriptions are our own reference summary and may differ by controller or software version — defer to FANUC documentation, or contact us to confirm. Henghuan is an independent service provider with no affiliation or endorsement by FANUC.