Alarm code ‘A’ on a Fanuc αi Power Supply Module (αiPS / PSM, A06B-6110 and A06B-6120 series) means the cooling fan on the external cooling fin — the radiator fan at the back of the module — has stopped. On the larger A06B-6110-Hxxx / A06B-6120-Hxxx units, the same alarm also covers the internal stirring fan (the small fan that circulates air inside the case). It is the counterpart of PSM alarm 2: ‘2’ is the internal control-side fan, ‘A’ is the heatsink side that dumps the real kilowatts. And because the PSM feeds the DC link for every amplifier on the machine, alarm A stops everything.
Where the radiator fan lives
αi amplifiers mount through the cabinet wall: the electronics face inward, while the finned heatsink pokes out the back into an external cooling duct. The radiator fan sits in that duct, pulling air across the fins — out of sight, which is exactly why it dies unnoticed: it breathes unfiltered shop air and nobody sees it stop.
Step 1 — Inspect both fans (15 minutes)
- Radiator fan: open the cabinet's rear duct cover and watch the fan with the machine powered — not spinning, twitching or crawling = confirmed. Power OFF and spin it by finger: gritty, notchy rotation means the bearings are gone even if it still ran.
- Stirring fan (A06B-6110/6120 large units): check the internal circulation fan too — on these models either fan raises the same alarm A.
- Dust and swarf: the duct-side fan is usually the dirtiest component in the machine — matted dust, oil mist, sometimes swarf pulled into the duct. Clean the fan, the duct and the heatsink fins (power off). If the fan spins up after cleaning, plan a replacement anyway — a stalled fan has stressed windings and will strand you again.
- Connector: reseat the fan's connector; check for brittle, oil-hardened wires at the crimp.
Step 2 — Replace the fan unit
- The radiator fan is a bolt-in unit in the duct; the stirring fan is a cassette in the module. Use the correct Fanuc fan for your PSM model (spec per the unit's parts list/nameplate) and match the airflow direction arrow.
- Generic two-wire fans keep the alarm on: the module monitors fan rotation via a sense signal — a fan without the sense wire reads as stopped even at full speed. This is the most common reason a ‘new fan’ fails to clear a fan alarm.
Step 3 — If both fans are genuinely healthy: the loose-board case
Fanuc's own manual lists a cause people never suspect: poor installation of the control printed-circuit board. The PSM's control PCB (behind the faceplate) mates to the power board through board-to-board connectors — if the faceplate/board is not pushed fully home, or a connector between control and power boards has worked loose, the fan-sense signal path is broken and the module reports alarm A with perfectly good fans.
- Power OFF, lock out, and wait for the charge LED to go out.
- Press the faceplate/control board firmly home as far as it will go; check that no board-to-board connector is half-seated (common after a previous repair or transport vibration).
- If the alarm persists with good fans and seated boards, the fan power/sense circuit on the board has failed — a board-level repair we do routinely on 6110/6120-series PSMs.
Why you should not bridge or ignore it
The radiator fan is what carries the PSM's real heat load. Run without it and the heatsink temperature climbs until the overheat trip — or, worse, until the bus capacitors and rectifier age in fast-forward. A fan is a consumable; a heat-soaked PSM is a machine-down repair. Fix the fan, and while the duct is open, verify the cabinet filters and heat exchanger too.
Related guides
- Fanuc PSM alarm 2 (internal fan stopped)
- Fanuc αi PSM alarm 04 (single phase failure)
- Fanuc SVM alarm F (amplifier fan failure)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained