Fanuc spindle alarm 79 occurs only with Dual Check Safety, and means an abnormal operation was detected during the initial test — the self-verification sequence the safety function runs at start-up did not behave as expected. It is the catch-all of the DCS power-up family: where AL-74 names the CPU test and AL-75 the CRC test, AL-79 reports the initial test sequence itself going wrong.
Fanuc's remedy — with the standard preamble
The manual: replace the Spindle Amplifier or its control printed-circuit board. Before that, the same ten-minute preamble as its siblings — which regularly saves a board from an unnecessary trip:
- One clean power cycle: full off, charge LEDs out, on. A power dip mid-test aborts the sequence spuriously (see AL-10 for control-supply health). Once — a repeat is real.
- Reseat the control-board faceplate (power off, locked out) — a half-seated board-to-board connection disturbs the test environment; the standing first fix of this series.
- History check: AL-79 arriving alongside AL-74/75/76 confirms a failing board environment — one repair covers the family. AL-79 immediately after board/software work → verify board pedigree and software series/edition first.
The repair
A persistent AL-79 is a control-board fault in the safety test path — board-level repair (processor environment, storage, supply rails) or an exact-specification exchange. Acceptance, as for every safety-chain repair: a documented safety-function test, demonstrating the initial test completes and the safety monitoring intervenes correctly — not merely a clean boot.
Related guides
- Fanuc spindle alarm 74 (CPU test failed)
- Fanuc spindle alarm 75 (CRC test error)
- Fanuc spindle alarm 76 (safety function not executed)