This guide covers a run of six spindle amplifier alarm codes — 15, 16, 17, 18, 19 and 20 — that Fanuc's manual documents briefly but that split into two very different families: AL-15 is a machine-side sequence fault you can often fix in the cabinet, while 16–20 are amplifier internal self-test failures where the useful knowledge is what to check before condemning the unit — and what a repairer must verify so the alarm doesn't come back.
AL-15 — Spindle / output switching sequence not completed
Applies to machines with spindle switching or output (winding) switching — typically lathes and machining centers whose spindle motor has low-speed/high-speed windings selected by external magnetic contactors, or machines sharing one amplifier between two spindles. The control raised a switch request (signal SPSL or RSL) and the confirming state-check signal (MCFN, MFNHG, RCH or RCHHG) did not transition within one second — the contactors did not do what was asked.
- The switching magnetic contactor itself: listen and watch during a switch command — no clunk means no action. Check the contactor's coil supply first (fuse, interposing relay); if supply is present and it still does not operate, replace the contactor. Burnt/welded main contacts also count as ‘inoperative’ — inspect them.
- The confirmation path: the contactor operated but the amplifier never saw it — faulty auxiliary contact on the contactor, broken wiring, or a dead input on the I/O unit. Trace the MCFN/RCH signal from the auxiliary contact through to the I/O module and watch it on the PMC diagnosis screen while commanding a switch.
- The ladder: if this machine's sequence was recently modified (retrofit, spindle work), the PLC may simply not complete the handshake within 1 second — timing must be corrected in the ladder.
AL-16 — CPU RAM error found by Dual Check Safety
The safety function's independent RAM test failed — same family as AL-13, caught by the redundant safety channel instead. One clean power cycle (rule out a brown-out boot), then it is a control-board fault: board-level repair or exchange. Because this involves the Dual Check Safety path, insist on a repairer who load-tests and verifies the safety function — not just ‘it powers up’.
AL-17 — Error in amplifier ID data
The unit cannot read its own identity data correctly. Fanuc's own first check: is the control printed-circuit board mounted correctly? A loose control board breaks the connection to the power unit's ID storage. Power off, lock out, push the faceplate fully home — the same fix as several alarms in this series. Board seated and AL-17 persists → the ID circuit or board interconnect has failed — repair or exchange. (Its sibling AL-14 is the ID being readable but wrong — usually a mixed-board repair; AL-17 is the ID being unreadable.)
AL-18 — Sum check abnormal
The program/data ROM checksum failed — corrupted software storage on the control board. Fanuc: replace the amplifier or its control PCB. Bench reality: sometimes reflashable/repairable at board level, otherwise a correct-specification replacement board (exact model and software series/edition — see the AL-14 warnings about mismatched boards).
AL-19 & AL-20 — U-phase / V-phase current detection offset too high
At every power-up the amplifier zeroes its current sensors; if the phase-U (AL-19) or phase-V (AL-20) detection circuit shows excessive offset with no current flowing, the sensing chain has drifted or failed — and running with bad current feedback risks rough motor behaviour and misleading overcurrent trips.
- The one case you can fix yourself — straight from the manual: if AL-19/20 appears immediately after the control PCB was replaced, the board-to-power-unit connectors are not fully plugged. Power off and verify every interconnect between the control board and power unit is seated.
- Otherwise: the current sensor or its input stage has failed — amplifier repair (sensor replacement and offset verification under load) or exchange.
Diagnosis checklist (in order)
- AL-15: machine-side — contactor coil supply → contactor → auxiliary contact/wiring/I-O on the PMC screen → ladder timing. Not an amplifier fault.
- AL-16/17/18: one clean power cycle → reseat the control board faceplate → then board-level repair or exchange with exact-spec verification.
- AL-19/20: just replaced the control PCB? → reseat the board-to-power-unit connectors. Otherwise → current-sensing repair.
- For every internal alarm (16–20): record the amplifier's full A06B number and software series/edition before sending it anywhere.
Related guides
- Fanuc spindle alarms 13 & 14 (CPU RAM / ID mismatch)
- Fanuc spindle alarm 12 (DC link overcurrent)
- Fanuc αi PSM alarm 04 (single phase failure)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained