The FANUC 21i series (21i-TA/TB for lathes, the 210i/210is variants and its close relative the 18i) is a very common control family on mid-range machines. Its alarm logic follows the classic FANUC structure, but the alarm map of this series is divided in more detail: servo alarms live in two separate ranges, rigid tapping and spindle alarms have blocks of their own, and there are extended P/S alarms above 5000. On this page you will find only the structure and differences specific to 21i / 21i-TB; for the map of alarm groups shared by all series, see our main guide: FANUC Alarm Codes and Solutions.
The 21i Alarm Map: How the Ranges Are Divided on This Series
| Range | Group |
|---|---|
| 000–255 | P/S alarms (program errors) — shown as “xxxBP/S” during background editing |
| 300–349 | APC (absolute pulse coder) alarms — shown with the axis number |
| 350–399 | SPC (serial pulse coder) alarms — 360s built-in encoder (INT), 380s external scale (EXT) |
| 400–499 | Servo alarms (1/2) — control side |
| 500–599 | Overtravel alarms |
| 600–699 | Servo alarms (2/2) — drive/amplifier side (a block specific to this family) |
| 700–739 | Overheat alarms |
| 740–748 | Rigid tapping alarms |
| 749–799 | Spindle alarms |
| 900–999 | System alarms |
| 5000+ | Extended P/S alarms (e.g. 5010) |
APC Alarms (300–309): Per-Axis Battery and Encoder Diagnosis
On the 21i, APC alarms arrive with the axis number (e.g. “APC ALARM: 2 AXIS”) — they tell you directly which axis encoder is in trouble. The critical trio are the battery alarms: 307 (BATTERY DOWN 1) says the battery is weakening — the machine keeps running; replace the battery as soon as possible with the control switched ON. 306 (BATTERY ZERO) means the voltage dropped below the level where data can be held — position is lost, and after the battery change alarm 300 (NEED ZRN) will ask for reference return on the axes. The 301–305 range are communication errors (cable, encoder or interface module); 309 says you attempted reference return without ever rotating the motor — turn the axis at least one revolution, power cycle and retry.
SPC Alarms (360–387): Built-In Encoder or External Scale?
This series has a subtlety that makes diagnosis easier: in serial pulse coder alarms, the (INT) tag at the end of the message points to the encoder built into the motor, while (EXT) points to the external scale (linear scale / separate detector). The 360–369 range covers checksum, phase, count and communication errors of the built-in encoder; 380–387 are the external-scale counterparts of the same faults. On closed-loop machines with scales, if you see an EXT alarm, look at the scale, the reader head and the scale cable — not the motor. This distinction saves hours of searching in the wrong place.
The Second Servo Block (600–699): The Drive Side
One of the most important differences from older series: while the 400s report control-side servo errors, the 600s report faults of the drive (SVM/SVU) and power supply module (PSM) directly. The ones we meet most on the shop floor:
| Code | Meaning and how to approach it |
|---|---|
| 600 | DC link overcurrent: possible short circuit or strain at the drive output or motor. Do not keep resetting before the motor cable and winding resistances are checked. |
| 601 / 606 | Cooling fan failure (601 drive heat-sink fan, 606 power-module external fan): clean or replace the fan; running a drive with a dead fan invites 602 (overheat). |
| 602 / 603 | Drive overheat / IPM (intelligent power module) overheat alarm: check cabinet ventilation, load and fans; if it repeats after cooling, the drive goes to service. |
| 604 | Communication fault between SVM and PSM: check the inter-module ribbon/communication cables. |
| 605 / 607 | 605: regenerative power too high (heavy braking load); 607: one input power phase lost — check the mains supply and fuses. |
Rigid Tapping (740–748) and Spindle Alarms (749–799)
On machines cutting threads synchronously, 740/741 (EXCESS ERROR) say the spindle position deviation exceeded its limit — look for a dull tap, a wrong S/F match or spindle position-gain issues; 742 is an LSI overflow. In the spindle block, 750 (SPINDLE SERIAL LINK START FAULT) reports that the spindle control was not ready at power-up (check drive power and the power-up sequence), 752 (MODE CHANGE FAULT) that a switch between Cs contouring / spindle positioning / rigid tapping modes did not complete, and 754 (ABNORMAL TORQUE) that abnormal load was detected on the spindle motor — with 754, rule out mechanical causes such as a jammed tool or chuck first. Remember that rigid-tap program errors (200–204) are shared across all series — they live in the main guide and the P/S list.
Two Subtleties Worth Knowing on the 21i
1) BP/S alarms: errors made while editing a program in the background appear as “xxxBP/S” on the input line of the editing screen instead of the normal alarm screen, and clear with any MDI key — they do not stop the machine and need no panic. 2) P/S alarms above 5000: program errors of extended functions (e.g. 5010); do not mistake the large number for a hardware fault — the fix is still in the program. The alarm history screen exists on this series too; see FANUC Alarm, Message and Diagnosis Screens for how to use it.
Frequently Asked Questions
Can I keep machining with battery alarm 307?
Yes — 307 (BATTERY DOWN 1) is the warning stage and the machine keeps running; but replace the battery within days, with the control powered ON. If you wait, it degrades to 306 (BATTERY ZERO), position is lost and every axis must be re-referenced.
I see both a 4xx and a 6xx servo alarm for the same axis — which one do I chase?
Chase the 6xx first: drive-side faults (overcurrent, fan, overheat, phase loss) are usually the root cause and the 4xx alarm is their consequence. The 4xx will not clear until the physical cause addressed by the 6xx is removed.
Are APC alarms 301–305 battery alarms on a 21i?
No. 301 is a communication error, 302 a time-out, 303 a framing error, 304 parity and 305 a pulse error; each names the axis as “n AXIS”. The cause is the encoder (APC), its cable or the servo interface module, not the battery. Check that axis’s encoder cable and connectors first, then power off and on; if it recurs the encoder is a service job. The battery alarms are 306, 307 and 308.
What does alarm 309 (ZRN IMPOSSIBLE) mean?
A reference return was attempted before the motor had turned at all. The fix is simple: jog the axis far enough for the motor to turn one revolution, switch the machine off and on, and repeat the reference return.
What is the difference between 401 (V READY OFF) and 404 (V READY ON)?
401: the drive’s ready signal (DRDY) has dropped. Check the emergency-stop chain, the power supply module’s control power, the main contactor (MCC) and any alarm code on the drive; 401 is usually the consequence of a drive alarm in the 6xx block. 404: the drive reported ready before the CNC asked for it (MCON), or the signal stayed on when power was removed. Here the cause is not the surroundings but the servo drive or the axis card in the CNC; service is needed.
What do I check first on 410 and 411 (EXCESS ERROR)?
410 is the position error exceeding its limit while the axis is stopped, 411 while it is moving. First checks: mechanical binding on the axis (packed chips, a locked way, binding after a crash), the motor-to-screw coupling, a loose encoder cable. If 411 appears only at high speed, suspect the servo gain or acceleration setting; if it appears at every speed, suspect the drive or motor. Before clearing the alarm and trying again, make sure the axis moves freely by hand (JOG at low override).
What do 409 (TORQUE ALM) and 754 (SPINDLE ABNORMAL TORQUE) mean?
Both are “abnormal load” alarms: 409 on a servo axis, 754 on the first spindle (764 on the second). Think of a jammed tool, the chuck hitting the part, a sudden rise in cutting load or a mechanical obstruction. Do not reset the alarm and restart the program from the same point; find the mechanical cause first.
Why are 413 (LSI OVERFLOW) and 415 (EXCESS SHIFT) counted as parameter errors?
413 is an overflow of the axis’s error register and usually comes from a wrongly set parameter. 415 is a speed command above 524,288,000 units/s and is mostly seen when CMR (the command multiplier) is wrong. Both are solved by undoing the last parameter change; if no parameter was changed, call service.
Which parameters does alarm 417 point to on a 21i?
The same digital servo parameters as on the 0i: 2020 motor type, 2022 rotation direction (111 or -111), 2023 velocity feedback pulses, 2024 position feedback pulses and the servo axis number. The alarm appears when one of these is out of range or has been changed; the fix is to undo the change or repeat the servo initial setting.
What should I do on overheat alarms 700, 701 and 704?
700, control unit overheat: check that the fan turns and clean the air filter. 701, the fan motor on top of the cabinet has overheated or stopped: check it and replace if necessary. 704, spindle overheat detected through spindle speed fluctuation: lighten the cut if the load is heavy, check whether the tool is sharp; if those are normal, suspect the spindle drive.
Which connection do I look at for serial spindle alarms 749 and 750?
749 is a communication loss during operation: check that the optical cable between the CNC and the spindle drive is connected, unbroken and seated; if the cable is fine, the main CPU board or the drive board is suspect. 750 is the drive not being ready at power-on: the optical cable may be wrongly fitted or the spindle drive may be unpowered; do not power up the CNC while the drive LED shows an error code. Detail is shown in the SPE/S2E/S1E/SHE bits of DGN 409.
Is alarm 5010 (END OF RECORD) a hardware fault?
No, it is a program error: the program reached the end-of-record mark (%) without an end command (M02/M30/M99). Add M30 at the end of the program (M99 in a subprogram). All alarms above 5000 are extended P/S alarms; the fix is in the program.
What if I cannot clear an alarm with RESET on a 21i?
The cause is still active. Battery alarms (306/307), overheat and drive alarms will not clear until the physical cause is removed. System alarms (900s) and some servo alarms need a power cycle. If the alarm returns after a power cycle, note the sequence from the alarm history screen (SYSTEM → alarm history) and pass it to service.
Other series guides: If your machine runs a 0i / 0i-Mate control, see our FANUC 0i / 0i-Mate Alarms page.
On the operator side: For what to do about each cycle, overtravel, servo, spindle and battery alarm an operator actually meets, see our FANUC Operator Alarms page. If the alarm comes from a programming error, the FANUC G and M Code List also helps.