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.
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.