What the SRVO-204 alarm means

The external emergency stop input is active, or the two channels of the external E-stop circuit disagree. This fault appears in a large share of our FANUC R-30iA / R-30iB / R-30iB Plus service tickets, and the first round of on-site checks usually finds the source.

Most common causes

  • External E-stop device (remote button, safety PLC output) active
  • Only one of the two channels wired or one channel failed
  • Timing mismatch between channels beyond tolerance
Field note: Most of the time the problem is the first item on the list, so ruling it out saves you half the time.

Step-by-step field fix

  1. Check the external E-stop devices and reset them
  2. Verify both channels of the external E-stop are wired and switching together
  3. Check for welded relays in the external safety circuit
Safety first: For FANUC srvo-204, this alarm is the safety circuit itself: jumpering a door switch or forcing a contactor removes the protection entirely - no protective action when something happens, and no defensible account of responsibility. On site you may look at after confirming power-off the first step yourself: Check the external E-stop devices and reset them. Beyond that it is the e-stop chain and safety circuit work - leave it to a professional.

How we diagnose safety-chain faults on site

FANUC srvo-204 belongs to the safety chain & e-stop family. On FANUC srvo-204 the failing stage is Dual Check Safety (e-stop board plus dual-channel safety circuit) on the R-30iB Plus / R-30iB Mate Plus / R-30iA - continuity of one series loop: e-stop buttons, door switches, light curtains, safety relays and the enabling device. We start reading FANUC srvo-204 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-204 runs: e-stop / door / curtain -> safety relay (dual channel) -> controller safety input -> motor-off and brake apply.

On FANUC srvo-204 the field usually reports External E-stop device (remote button, safety PLC output) active, but the check that matters is not continuity: The point of a safety loop is not whether it conducts but whether both channels conduct together. The relay compares channels; if one opens tens of milliseconds early it latches as a channel fault. The commonest causes are plain: oxidised door-switch contacts raising contact resistance, or one loose terminal. We measure segment by segment - pendant e-stop, panel e-stop, external e-stop, safety door - and lock onto whichever segment reads differently on the two channels.

Field notes on this page cover FANUC srvo-204 (External E-Stop (SVEMG Abnormal)); written by the Henghuan repair team, updated 2026-10-01.

Real Field Case

Machine: FANUC R-30iB Plus (2021)

Symptom: A robot raised SRVO-204 (external E-stop fault) even though no one had pressed any E-stop.

What we did: We inspected the external E-stop wiring and found one terminal loosened by vibration; we re-crimped and secured it.

Result: The alarm cleared after the wiring fix and we advised adding that terminal to the inspection checklist; about 1.5 hours.

No luck? Hand it to the team that fixes this daily

Send the alarm code, the robot model and a description of what was happening at the alarm. Our engineers handle FANUC srvo-204 FANUC srvo-204 every day, and remote diagnosis by phone/video is free.

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Related Safety Chain & E-Stop alarms

SRVO-105 — Door Open or E.Stop SRVO-201 — Panel E-Stop SRVO-202 — TP E-Stop SRVO-230 — Chain 1 (+24V) Abnormal SRVO-231 — Chain 2 (0V) Abnormal SRVO-232 — NTED Input Abnormal
Same family: this code belongs to the Safety chain & e-stop alarms family - the page collects all Safety chain & e-stop-family codes we have handled, with the common inspection order and spares.
Other brands: see our KUKA, ABB, Yaskawa and Universal Robots field-fix libraries — or the full 10-brand hub.

Frequently asked questions

QWhat does the FANUC srvo-204 alarm mean?

The external emergency stop input is active, or the two channels of the external E-stop circuit disagree. This fault appears in a large share of our FANUC R-30iA / R-30iB / R-30iB Plus service tickets, and the first round of on-site checks usually finds the source. FANUC srvo-204 sits in the safety chain & e-stop family; on our work orders the highest-frequency trigger for this code is "External E-stop device (remote button, safety PLC output) active".

QCan I clear a FANUC srvo-204 alarm myself?

Whether FANUC srvo-204 clears on its own depends on the outcome of "Check the external E-stop devices and reset them". Finish the flow with "Check for welded relays in the external safety circuit" to confirm FANUC srvo-204 - do not skip it. If that step is clean, the second most frequent cause on FANUC srvo-204 is "Only one of the two channels wired or one channel failed". Do not chase takt through a repeating FANUC srvo-204. Left alone, safety chain & e-stop faults tend to graduate from repairable to must-replace.

QA safety-chain fault like FANUC srvo-204 stops the line - how fast can you be here?

The commonest causes we see on FANUC srvo-204 are "External E-stop device (remote button, safety PLC output) active, Only one of the two channels wired or one channel failed, Timing mismatch between channels beyond tolerance", so we stock the van against those. For FANUC srvo-204 specifically, we stock along the loop: safety relays, common door switches, e-stop buttons, light-curtain pairs and terminal blocks. For FANUC srvo-204, a safety-chain fault stops the whole line, so we rank it top priority and pre-triage by phone: door switch, e-stop or relay. Before we roll, tell us what FANUC srvo-204 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.

This page on FANUC srvo-204 was written by the Henghuan Robotics repair team from field service experience. Alarm descriptions are our own summary for troubleshooting reference and may vary by controller and software version — when in doubt, check your FANUC documentation or contact us. Henghuan is an independent service provider, not affiliated with or endorsed by FANUC.