The short answer
A servo encoder error means the drive stopped receiving valid position data from the motor encoder. It is A.C90 on Yaskawa Sigma-7, AL.16/AL.20 on Mitsubishi MR-J4 (AL. 016/AL. 020 on MR-J5), FLT S45 on Kinetix 5700, SV0368 on FANUC controls and F31110 on SINAMICS V90. In most cases the cause is the encoder cable, connector or noise, not the motor. The table below maps eight common alarm families across brands, every code taken from the maker’s own manual.
- Mitsubishi separates encoder errors at power-up (AL.16, initial communication) from errors while running (AL.20, normal communication). That timing tells you where to look.
- Yaskawa raises A.830 when the absolute-encoder battery falls to about 2.7 V. A.810 means the multiturn data was actually lost.
- FANUC SV0410 is excess position error at stop and SV0411 is excess error while moving. Kinetix 5700 uses FLT S54 – POSN ERROR for both.
- Third-party code lists conflict, especially across MR-J2S, MR-J3, MR-J4 and MR-J5. Use the OEM manual for your exact drive generation.
| Alarm family | Yaskawa Sigma-7 | Mitsubishi MR-J4 / MR-J5 | Kinetix 5700 | FANUC CNC with αi/βi amplifiers | Siemens SINAMICS V90 PN |
|---|---|---|---|---|---|
| Encoder communication error | A.C90 Encoder Communications Error | AL.16 / AL. 016 Encoder initial communication error 1; AL.20 / AL. 020 Encoder normal communication error 1 | FLT S45 – FDBK COMM FL (Motor Feedback Data Loss); FLT S43 – FDBK LOSS FL | SV0368 SERIAL DATA ERROR(INT); SV0369 DATA TRANS. ERROR(INT) | F31110 Serial communications error |
| Encoder battery / absolute position lost | A.830 Encoder Battery Alarm; A.810 Encoder Backup Alarm | AL.9F / AL. 09F Battery warning; AL.25 / AL. 025 Absolute position erased | FLT M06 – FDBK BATTERY LOW; FLT M05 – FDBK BATTERY LOSS | DS0307/DS0308 APC ALARM: BATTERY LOW 1/2; DS0306 APC ALARM: BATTERY VOLTAGE 0; DS0300 NEED REF RETURN | See manual |
| Overcurrent | A.100 Overcurrent Detected | AL.32 / AL. 032 Overcurrent | FLT S10 – INV OVERCURRENT | SV0438 INV. ABNORMAL CURRENT; SV0449 INV. IPM ALARM | F30001 Power unit: Overcurrent |
| Overload | A.710 Instantaneous Overload; A.720 Continuous Overload | AL.50 / AL. 050 Overload 1; AL.51 / AL. 051 Overload 2 | FLT S07 – MTR OVERLOAD FL; FLT S13 – INV OVERLOAD FL | SV0436 SOFTTHERMAL(OVC) | F30005 Power unit: Overload I2t; F7011 Motor overtemperature |
| Overvoltage (regeneration) | A.400 Overvoltage; A.320 Regenerative Overload | AL.33 / AL. 033 Overvoltage; AL.30 / AL. 030 Regenerative error | FLT S35 – BUS OVERVOLT FL | SV0439 CNV. OVER VOLT DC LINK | F30002 DC link voltage, overvoltage |
| Undervoltage | A.410 Undervoltage | AL.10 / AL. 010 Undervoltage | FLT S33 – BUS UNDERVOLT FL (S34 user limit) | SV0433 CNV. LOW VOLT DC LINK; SV0435 INV. LOW VOLT DC LINK | F30003 DC link voltage, undervoltage |
| Following / position error excess | A.d00 Position Deviation Overflow | AL.52 Error excessive / AL. 052 Excessive error | FLT S54 – POSN ERROR | SV0410 EXCESS ERROR (STOP); SV0411 EXCESS ERROR (MOVING) | See manual (position loop normally in the controller) |
| Overspeed | A.510 Overspeed | AL.31 / AL. 031 Overspeed | FLT S03 – MTR OVERSPEED FL (S04 user limit) | See manual | F7901 Motor overspeed |
Why alarm lists online disagree, and which to trust
Search results for servo alarm codes are full of copied lists, and they conflict. A typical case is Mitsubishi AL.16: some lists describe it as a generic encoder fault in older MR-J2S wording, while the MR-J4 and MR-J5 manuals name it Encoder initial communication error 1 and split it into detail codes 16.1 to 16.F. Numbering also changes between generations: MR-J5 writes AL. 016 where MR-J4 writes AL.16. FANUC alarm numbers depend on the control series (SV0436 on 0i-D/0i-F-era controls, 436 on older 16i/18i/21i).
Use the troubleshooting manual for your exact drive generation and firmware. Each code in the table above is cited to the OEM manual: Yaskawa SIEP S800002 19O, Mitsubishi SH(NA)-030109ENG-T and SH(NA)-030312ENG-P, Rockwell 2198-UM002Q, FANUC B-64305EN/03 and Siemens A5E37208830. Where we could not confirm a code in a primary source, the table says "see manual". FANUC amplifier 7-segment LED codes are in the αi amplifier maintenance manual (B-65285EN), which we could not access, so the table gives the CNC SV/DS messages instead.
Encoder communication and absolute-position alarms
Physics: a serial encoder sends position as digital data frames over a shielded twisted-pair cable. The drive raises an alarm when frames stop arriving or fail their checks. Absolute encoders also keep a multiturn count, held by a battery when control power is off unless the encoder is battery-less.
Timing logic: an encoder alarm that appears at every power-up points to a connection that is open, miswired or shorted, or a dead encoder. Mitsubishi reports this as AL.16. An alarm that appears only while running, after the machine has been working, points to an intermittent fault such as a cable flexing in a drag chain, a cracked conductor, a loose shield or noise from motor power cables run alongside. Mitsubishi reports this as AL.20.
- Check the connectors at both ends for bent pins, contamination and full seating (with power locked out).
- Inspect the cable along its whole run, especially at drag-chain bends. If the fault follows motion, flex the cable gently while watching the alarm history.
- Check shield termination and separation from motor power cables. The MR-J5 and V90 manuals both list noise and shielding as causes.
- Swap in a known-good cable before condemning the motor. Mitsubishi lists amplifier and encoder failure only after the cable checks.
Battery and absolute position: replace a low battery before it reaches zero. Yaskawa and FANUC both specify replacing it with control power on so the encoder keeps its position. Mitsubishi says to replace the battery below 3 V DC. After a true loss (A.810, AL.25, DS0306, FLT M05), the machine must be re-homed or re-referenced before production. Check the result with a slow, supervised move.
Overcurrent, overload and overspeed
Overcurrent is a fast hardware trip on the output stage: A.100, AL.32, FLT S10, SV0438/SV0449 or F30001. If it trips the instant the servo turns on, suspect a short between motor phases or to ground in the motor or its cable, or a damaged power stage. If it trips only during hard acceleration, look at gains, acceleration settings and mechanical jams.
Overload is a thermal model, not an instant trip. Yaskawa’s manual describes A.710 as running for several seconds to several tens of seconds at a torque that largely exceeds the rating, and A.720 as continuous operation above rating. FANUC’s SV0436 is the software thermal (OVC). Common causes are a holding brake that is not releasing, binding ways or ballscrews, a duty cycle beyond the motor rating, or oscillation from poor tuning. Mitsubishi asks for at least 30 minutes of cooling after AL.50/AL.51 instead of repeated resets.
Overspeed (A.510, AL.31, FLT S03, F7901) means measured speed exceeded the limit. It usually comes from overshoot caused by aggressive tuning, a wrong electronic gear ratio, gravity driving a vertical axis, or wrong motor phase wiring after a repair.
- Check the load ratio or effective torque monitor in the set-up software.
- Confirm the brake releases: listen for the click and check brake voltage at the motor.
- With power locked out, turn the axis by hand where it is safe to do so and feel for binding.
- With the motor disconnected from the drive, measure phase-to-phase winding resistance for balance and insulation to ground with a megohmmeter.
DC bus alarms: overvoltage and undervoltage
During deceleration, or while lowering a vertical load, the motor acts as a generator and pumps energy into the DC bus. If the regenerative resistor or regenerative supply cannot absorb it, the bus voltage rises and the drive trips. Overvoltage alarms (A.400, AL.33, FLT S35, SV0439, F30002) that happen on deceleration point to the regeneration path: a missing, open or undersized resistor, a wrong regenerative-resistor parameter, a deceleration rate that is too fast, or high supply voltage. Regenerative overload (A.320, AL.30) means the resistor itself is running too hot over the duty cycle.
Undervoltage (A.410, AL.10, FLT S33, SV0433/SV0435, F30003) at power-up points to supply wiring, a blown fuse, a lost phase or a precharge problem. If it appears only under heavy acceleration, the supply is sagging under load. Measure line voltage on all phases while the fault occurs.
Following-error (position deviation) alarms
A following-error alarm fires when the gap between commanded and actual position exceeds a set limit: A.d00 (Yaskawa Pn520), AL.52, FLT S54 or SV0410/SV0411. The alarm describes a symptom. Find the cause by noting when it happens.
- At stop or standstill (FANUC SV0410): gravity load with a weak holding brake or too little holding torque, or a mechanical drift such as a slipping coupling.
- While moving (SV0411, FLT S54 during a move): binding, torque limit reached, acceleration too steep for the motor, or gains too soft.
- Immediately on the first move after maintenance: motor phase order, encoder mounting or a swapped motor and encoder cable.
- Only on long or fast moves: the limit itself may be too tight for the commanded speed. Raise it only after confirming the mechanics are sound.
Do not widen the following-error limit to make the alarm go away until you know why the axis is lagging. The limit protects against runaway and collisions.
Frequently asked questions
What does Yaskawa A.C90 mean?
A.C90 is Encoder Communications Error: the SERVOPACK cannot communicate with the encoder. Check the encoder cable and connectors, shield termination and routing away from power cables, then try a known-good cable. Only then suspect the encoder or the drive. Yaskawa’s alarm list marks A.C90 as not resettable by an alarm reset, so clear the cause and cycle power.
What is the difference between Mitsubishi AL.16 and AL.20?
AL.16 (Encoder initial communication error 1) occurs while the amplifier is establishing communication with the encoder at power-up. AL.20 (Encoder normal communication error 1) occurs after communication was working. AL.16 points to a missing or miswired connection or a dead encoder, and AL.20 to intermittent faults such as cable flexing or noise. MR-J5 uses AL. 016 and AL. 020.
What causes a servo following error?
The axis cannot keep up with the command. Usual causes are mechanical binding, a dragging brake, a torque limit, acceleration beyond the motor’s capability, soft gains, or a feedback problem. Note whether it happens at stop, while moving or on the first move after maintenance; the FANUC SV0410/SV0411 split shows why that matters.
How do I test a servo motor with a multimeter?
Lock out the drive and wait for the DC bus to discharge. Disconnect the motor from the drive, then measure phase-to-phase winding resistance on U-V, V-W and W-U; the three should match closely. Use a megohmmeter, not a multimeter, for insulation from each phase to ground. Never megger with the drive connected.
Why does my servo hum or vibrate at standstill?
A hum at standstill is usually the control loop reacting to a mechanical resonance or to gains set too high for the load inertia. Re-run the drive’s auto-tuning (Yaskawa tuning-less or autotuning, Mitsubishi one-touch or adaptive tuning, Rockwell load observer). Then check couplings and mounts for looseness and check that the inertia ratio is reasonable.
What do the LED codes on a FANUC αi servo amplifier mean?
The amplifier’s 7-segment codes are defined in FANUC’s SERVO AMPLIFIER αi series Maintenance Manual (B-65285EN). We could not access it, so we do not list the codes here. The CNC also shows the fault as an SV alarm, such as SV0438 for motor overcurrent or SV0436 for the software thermal, and you can diagnose from that.
Can encoder cable noise cause servo alarms?
Yes. The MR-J5 manual lists the surrounding environment, meaning noise, among the causes of AL. 016, and the V90 manual tells you to check encoder cable shielding for F31110. Route encoder cables away from motor power cables, terminate shields as the manual shows and use the maker’s cable.
Why do servo overload alarms keep coming back after reset?
Overload alarms come from a thermal model, so resetting does not cool the motor. Mitsubishi asks for at least 30 minutes of cooling after AL.50/AL.51 and warns against repeated resets. Find the source of the extra torque: a brake, binding, the duty cycle or tuning oscillation.