A FANUC power supply module (PSM) alarm of 1 means overcurrent in the main circuit, 2 a stopped cooling fan, 3 an overheated heat sink, 4 low DC link voltage, 5 a failed precharge and 7 DC link overvoltage. Alarm 6 depends on the generation: open input phase on alpha modules, low control power on alpha i. Only alarms 1 and 5 commonly mean the module itself has failed, and even those are worth ten minutes of isolation before you unbolt anything, because a shorted amplifier further down the rack produces exactly the same code.
The PSM is the one module every axis and the spindle depend on. It rectifies incoming three-phase power, charges the shared DC link, and on regenerative versions pushes braking energy back to the line. When it faults, the whole machine is down and every amplifier on the rack starts reporting alarms of its own — which is how plants end up shipping a perfectly good spindle amplifier. This guide is for the electrician standing in front of that cabinet.
Reading the module before you read the CNC
The PSM has its own status display and it is more trustworthy than the CNC screen, because it is the module describing itself rather than the control describing a symptom.
In normal operation the display shows a dash while the module is waiting — control power present, main contactor not yet closed — and a zero once the main contactor has pulled in and the DC link is up. A number with the alarm indicator lit is a fault. A display that is completely dark means no control power is reaching the module at all, which is a different problem from any alarm and usually a much cheaper one.
Write the number down before anyone cycles power. Then write down what the amplifiers show, because they will be showing something too.
The alarm table
As of October 2026 this is how the codes read on the two generations we see most. Confirm against the maintenance manual for your exact module, because FANUC revised the tables between the alpha and alpha i series and resistor-discharge versions add codes of their own.
| PSM code | Alpha PSM (A06B-6077 / 6087) | Alpha i PSM (A06B-6110) | Typical CNC message on i-series controls | Usually the PSM itself? |
|---|---|---|---|---|
| 1 | Overcurrent in main circuit / power module fault | Same | 437 CNV. OVERCURRENT POWER | Often — after isolating the rack |
| 2 | Control-circuit cooling fan stopped | Internal cooling fan stopped | 443 CNV. COOLING FAN FAILURE | Fan only |
| 3 | Main circuit heat sink overheat | Main circuit overload | 431 CNV. OVERLOAD | Rarely — look at airflow first |
| 4 | DC link voltage low | Same | 433 CNV. LOW VOLT DC LINK | Rarely — incoming power |
| 5 | DC link precharge not completed in time | Same | 442 CNV. CHARGE FAILURE | Often — or a short downstream |
| 6 | Input power abnormal (open phase) | Control power supply voltage low | 432 CNV. LOW VOLT CONTROL | Sometimes |
| 7 | DC link overvoltage | Same | 439 CNV. OVER VOLT POWER | Rarely — line or regeneration |
| A / E | Not used | External fan stopped / input open phase | 606 / 607 | Fan or incoming power |
On older controls paired with alpha-generation modules you will often see only a generic servo alarm and a V-ready-off, with the detail left on the module display. That is one more reason to read the module first. The axis-side version of this conversation is in FANUC alarm 401, 410 and 411.
Alarm 1 and alarm 5: the two that matter most
These are the codes behind most of the PSMs that arrive on our bench, and they share a diagnostic trick.
Alarm 1 means the module saw more current in its main circuit than it should, or its power module's own protection operated. Causes, in the order they turn up:
- A failed rectifier or switching device inside the PSM.
- A short across the DC link in a servo or spindle amplifier further down the rack.
- A gross incoming power problem — a badly unbalanced supply or a line disturbance.
- A failed current-detection or gate-drive circuit inside the PSM reporting a fault that is not there.
Alarm 5 means the module tried to precharge the DC link capacitors through its charging circuit and the bus never reached voltage in the time allowed. Causes:
- A failed precharge resistor or precharge relay circuit in the PSM. Repeated rapid power cycling is the classic way to cook one.
- A shorted or heavily loaded DC link somewhere else on the rack, so the bus cannot rise no matter what the PSM does.
- Aged bus capacitors.
- Low or missing incoming power on one phase.
Notice that item 2 appears on both lists. A shorted spindle amplifier output stage will hold the shared DC link down and make a healthy PSM report alarm 1 or alarm 5. So before pulling the PSM, isolate it: with power locked out and the DC link confirmed discharged with a meter, remove the DC link bus bars between the PSM and the first amplifier, leaving the PSM on its own. Restore control power and main power.
- If the PSM now comes up to its ready state, the PSM is fine and one of the amplifiers is dragging the bus. Reconnect them one at a time until the alarm returns — the last one you connected is your fault.
- If the PSM still alarms standing alone, the fault is in the PSM or upstream of it.
That test takes a competent electrician about twenty minutes and it is the difference between shipping the right module and shipping the wrong one. It is also why the spindle amplifier so often gets blamed for what is really a rack problem; we cover that module's own costs in FANUC spindle amplifier repair cost.
"Alarm 5 on a rack, first thing I do is pull the link bars. More often than not the supply comes straight up by itself and it is the spindle drive sitting there shorted. If you skip that step you are just guessing with somebody else's money."
— CNC field service technician, 21 years, machine tool distributor; name withheld by request
Alarm 4, 6 and 7: check the building before the module
Alarm 4, DC link voltage low, is far more often a real dip in incoming power than a failed module. Measure all three phases at the PSM input terminals, phase to phase, with the machine trying to run. A loose lug, a tired disconnect, a failing contact in the main contactor, or an undersized transformer feeding a 200 V class drive package from a 480 V plant bus will all produce it. If alarm 4 arrives during spindle acceleration specifically, suspect a supply that sags under load.
Alarm 6 on an alpha module is an open input phase: one of three line fuses, a contactor pole, or a conductor. Alarm 6 on an alpha i module is low control power — the separate single-phase feed that runs the module's own electronics — which sends you to that circuit's fuse and connector rather than the main input. Same number, different cabinet wiring, which is why the generation matters.
Alarm 7, DC link overvoltage, happens on deceleration, when the motors are returning energy to the bus. On a regenerative PSM that energy is supposed to go back to the line; if the line is high already, or its impedance is high because of a long run or a small transformer, the bus rises instead. Check that the incoming voltage is inside the module's rated range, and note whether the alarm correlates with hard spindle stops.
Voltage quality matters more than a single meter reading suggests. The U.S. Department of Energy's motor-systems guidance (energy.gov) notes that current unbalance in a three-phase circuit typically runs 6 to 10 times the percentage voltage unbalance, so a supply that looks only slightly off can be loading one leg of the rectifier far harder than the others. Recommended practice for power quality and harmonic limits at the point where a drive connects to the plant system is set out in IEEE 519 (ieee.org).
Alarm 2 and alarm 3: the cheap ones
A stopped fan and an overheated heat sink are the alarms you want. Fans are wear items. Replace the fan, clean the heat sink fins and the cabinet filters, check the cabinet cooler is actually cooling, and the alarm is usually gone.
Do not run past them. The rule of thumb for electrolytic capacitors is that every 10 °C of sustained temperature rise roughly halves their service life, so a PSM run hot for a year behind a dead fan has spent a good part of its bus capacitor life even if it never tripped again. An alarm 3 that returns with a working fan and a clean heat sink is telling you about duty cycle, ambient temperature or an internal thermal sensor — and at that point the module is worth a bench look.
Do this safely or do not do it
Every check above involves a cabinet that stores energy. The DC link capacitors in the PSM and in every amplifier stay charged after the disconnect is opened. The product safety standard for these drives, IEC 61800-5-1 (iec.ch), requires a warning label wherever stored charge takes longer than 5 seconds to fall below 60 V, and FANUC's label states the wait. Follow it, then prove the bus dead with a meter rated for the voltage before a wrench goes near a link bar.
OSHA's control-of-hazardous-energy standard (osha.gov) is, by the agency's own estimate, responsible for preventing around 120 fatalities and 50,000 injuries each year, and workers injured in hazardous-energy incidents lose an average of 24 workdays recovering. Live measurements at the input terminals fall under the energized-work rules of NFPA 70E (nfpa.org), which sets the arc-flash boundary where incident energy drops to 1.2 cal/cm². If your site requires an energized work permit for those readings, get one. A spindle drive is replaceable.
When to ship the PSM, and what it costs
Send the power supply module when:
- It alarms 1 or 5 standing alone with the DC link bars removed.
- It trips the main breaker or blows input fuses at power-up.
- Its display is dark with verified control power at its connector.
- It shows intermittent low-voltage alarms that you have ruled out at the incoming supply.
- A fan or heat-sink alarm returns with a new fan and clean airflow.
FANUC power supply module repair is quote-track at our lab: the bench evaluation is free, typical repairs run $500–$1,200, and no work proceeds without your approval of the firm quote. The repair covers the rectifier and precharge circuit, DC bus capacitor bank replacement, the control power supply, fans and a full recap, and it carries a 24-month warranty. Standard turnaround is 3–5 business days in lab; rush service is $149 for 1–2 business days.
What we cannot fix from Arlington is the incoming three-phase supply at your machine. If the evaluation finds a healthy module, we tell you that and tell you what to check, and the evaluation costs you nothing.
For how this module's price compares with the rest of the power-supply world, see industrial power supply repair cost; to confirm which module you are actually holding, the A06B part-number guide decodes the label. When you are ready, request a quote with the PSM code, the amplifier codes and the machine make and model, and follow the mail-in instructions.
Frequently asked questions
What does alarm 1 on a FANUC power supply module mean?
Alarm 1 means overcurrent in the PSM's main circuit, or its power module's protection has operated. The cause is either a failed power device inside the PSM or a short on the shared DC link in a downstream amplifier. Disconnect the DC link bars and power the PSM alone: if the alarm clears, an amplifier is at fault; if it stays, the PSM is.
What does alarm 5 on a FANUC PSM mean?
Alarm 5 means the DC link failed to precharge within the allowed time. The usual causes are a failed precharge circuit in the PSM, a shorted amplifier holding the bus down, aged bus capacitors, or a missing input phase. The same isolation test used for alarm 1 separates a PSM fault from a rack fault.
Is alarm 4 on the PSM a bad power supply module?
Usually not. Alarm 4 is low DC link voltage, and the most common cause is a real dip or imbalance in the incoming three-phase supply — a loose connection, a worn contactor, or a transformer that sags under spindle acceleration. Measure all three phases at the PSM input under load before suspecting the module.
Why does alarm 6 mean different things on different FANUC power supplies?
Because FANUC changed the alarm table between generations. On alpha modules such as the A06B-6077 and A06B-6087, alarm 6 is an open input phase. On alpha i modules such as the A06B-6110, alarm 6 is low control power supply voltage, and open phase is reported separately. Always check the manual for your module series.
Every axis and the spindle alarmed at once. Which module do I send?
Start with the power supply module. When the whole rack faults together, the shared PSM or the incoming power is the common factor, not several amplifiers failing simultaneously. Read the PSM display first, run the DC link isolation test, and send the PSM only if it alarms on its own.
How much does FANUC PSM repair cost?
Typical FANUC power supply module repairs run $500–$1,200 after a free bench evaluation, with a firm quote before any work. Standard turnaround is 3–5 business days in lab and the repair carries a 24-month warranty. If the module tests healthy, the evaluation costs nothing.
Can I keep running with a PSM fan alarm?
No — treat it as a stop-and-fix. A stopped fan lets the heat sink and bus capacitors run hot, and sustained heat shortens electrolytic capacitor life sharply. A fan is a small repair; a PSM cooked behind a dead fan is a much larger one.
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