Alarm 04 on a Fanuc αi Power Supply Module (PSM) means the module has detected a missing or severely low input phase — one of the three incoming phases (L1/L2/L3) is not delivering. The PSM shows 04 on its 7-segment display, and the CNC screen shows a spindle/servo alarm pointing at the power supply (the servo and spindle amplifiers drop because their shared DC link source is compromised). The good news: this is one of the most electrician-friendly Fanuc alarms — the cause is nearly always upstream wiring or switchgear, not the module. The bad news: running long on two phases cooks rectifiers. Diagnose promptly.
What is happening
The PSM rectifies 3-phase 200 V (or 400 V for PSM-HV) into the ~300 V DC link that feeds every SVM and SPM on the machine. It monitors the input; if one phase disappears or sags far below the others, rectifier ripple soars and the module raises alarm 04 and shuts down rather than carry full load on two phases.
Where the phase goes missing — checklist from the wall inward
- Measure at the PSM input terminals (L1-L2, L2-L3, L1-L3) with the machine trying to run. Two pairs healthy (~200 V) and one pair low/dead → confirmed open phase; note WHICH pair to guide the hunt upstream. All three healthy at the terminals with the alarm still present → skip to the PSM section.
- Line fuses / circuit protector: most αi cabinets fuse each phase ahead of the PSM. One blown fuse = textbook alarm 04. But a fuse blows for a reason — before replacing, megger downstream (PSM input, spindle transformer if fitted) so the new fuse does not follow the old one.
- Main contactor (MCC): a burnt or pitted contact on one pole passes voltage at no load but collapses under current — the sneakiest version, giving alarm 04 only under spindle load. Thermal-scan or measure across each pole under load; replace the contactor, not just the coil.
- Input reactor / line filter and terminals: loose or corroded lugs at the reactor, isolation transformer or terminal blocks — look for discoloration and re-torque. Vibration loosens these over years.
- Upstream of the machine: plant breaker with one weak pole, a lost phase on the feeder, or a failed changeover switch after generator operation. If other machines on the same feeder also misbehave, the machine is innocent.
If the input measures healthy — the PSM itself
- Phase-detection circuit fault: the sensing resistors/board inside the PSM can fail and report a phantom open phase — the alarm follows the module even on a known-good supply. Board-level repair.
- Rectifier module damage: a PSM that ran on two phases for a while may have a stressed rectifier — sometimes surviving as intermittent 04/07 alarms and a growling module. Bench testing settles it.
- Connector/ribbon issues between the PSM's control board and power stage after previous repair work.
After the fix — verify properly
- Measure all three phase pairs at the PSM terminals under full spindle acceleration — balanced within a few percent.
- Check DC link voltage on the diagnosis/amplifier display (~300 V DC steady).
- Run a heavy cycle and re-scan the contactor and terminals for heating — a repaired lug that warms up again was not the whole story.
Can you keep running?
No. Unlike a nuisance alarm, an open phase forces the remaining rectifier legs to carry ≈50% extra current — heat that kills the PSM's diode/IGBT modules and takes the machine down hard. The alarm exists precisely to stop that. Find the phase, fix the connection, then run.
Related guides
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained
- βi Combi drive alarm 51 (DC link undervoltage)
- Fanuc spindle alarm 12 (DC link overcurrent)