FANUC Alarm 401, 410 and 411: Amplifier, Motor or Neither?

FANUC alarm 401 means the amplifier never returned its ready signal — suspect the power supply module, the MCC contactor, the emergency-stop chain or the amplifier itself. Alarms 410 and 411 mean excess position error, which is usually mechanical or a drive that is not producing torque rather than a dead amplifier. Alarm 414 is not a diagnosis at all: it is a pointer telling you to read the servo diagnostic bits. Getting that distinction right before you pull anything is worth several hundred dollars and most of a shift.

This guide is for the person standing at the control with the machine faulted and a supervisor asking how long. It ranks the causes in the order they actually occur, gives the checks a plant electrician can do safely, and says plainly when the amplifier needs to come out.

What each alarm is telling you

Alarm	Reads as	What it literally means	First suspicion
SV0401	V READY OFF	CNC asked the drive to be ready; the ready signal never came back	Power supply module, MCC, E-stop chain
SV0410	Excess error, stopped	Position deviation while the axis is stationary exceeded its parameter	Axis not holding: brake, bind, no torque
SV0411	Excess error, moving	Position deviation during motion exceeded its parameter	Drive not delivering commanded torque
SV0414	Digital servo alarm	A fault was detected — the detail is in the diagnostic bits	Read DGN 200/201/204 before anything
SV0400 / 430	Overload / motor overheat	Thermal protection operated	Cooling, duty cycle, dragging axis

Two of these are frequently misread, and both misreads cost money.

401 is not automatically a dead amplifier. It is the absence of a signal. Anything in the chain that prevents the drive becoming ready produces it — including a power supply module that never came up, a main contactor that did not pull in, an open emergency-stop or overtravel circuit, or a communication fault between the CNC axis card and the amplifier. On multi-axis machines a 401 on every axis at once points hard at the shared power supply module or the MCC, not at three simultaneously dead amplifiers.

410 and 411 are usually mechanical. Excess position error means the axis is not where the CNC told it to be. If the drive were dead you would normally get a different alarm; excess-error faults are much more often a brake that did not release, a way or ballscrew that is binding, a loaded axis being asked for more than it can deliver, or parameters that were lost and restored wrong. The amplifier is on the list, but it is not the top of it.

The diagnostic step most people skip

Alarm 414 exists specifically to hand you to the diagnostic screen, and the diagnostic screen is where the machine names its own fault. Diagnostic numbers 200, 201 and 204 carry the digital servo detail as individual bits — overcurrent, overvoltage, overheat, feedback disconnect, pulse-coder faults and so on, per axis.

Write those bits down before you clear the alarm. They survive far less well than people assume, and a technician who calls the shop with "alarm 414 and DGN 200 bit 5 set" gets a different, faster conversation than one who calls with "the servo alarm."

The same discipline applies to the amplifier's own indicator. FANUC amplifiers display their own status and alarm codes on a small on-board indicator that is independent of the CNC screen. Record what it shows while the fault is live. A CNC alarm tells you what the control noticed; the amplifier indicator tells you what the drive thinks happened to itself, and when the two disagree that disagreement is itself diagnostic.

"New guys clear the alarm to see if it comes back, then call me with nothing. The bits and the amp LED at the moment it faulted are ninety percent of the diagnosis. Clear it and you have thrown that away and you are now waiting for it to fail again."

— CNC maintenance technician, 19 years, aerospace machining; name withheld by request

Ranked causes and the checks for each

For a 401, in the order they actually turn up:

1. Power supply module not ready. On alpha and alpha i systems the PSM feeds the DC bus for every axis. If it has not come up — blown input fuse, failed precharge, tripped thermal — no amplifier can report ready. Symptom that gives it away: every axis faults together.
2. MCC / main contactor. The contactor that closes main power to the drive section may not be pulling in. Listen for it, then check its coil circuit.
3. Emergency stop and overtravel chain. An open E-stop loop, a tripped overtravel limit or an unlatched guard interlock all hold the drive section off. This one is embarrassing frequently enough that it is worth ruling out in the first five minutes.
4. Control power to the amplifier. The amplifier needs its own control supply before it can report anything. No indicator on the amp at all points here.
5. Interface cabling. The connection between the CNC axis card and the amplifier carries the ready handshake. Vibration and coolant do what they do.
6. The amplifier itself. Genuinely common, but sixth on the list rather than first.

For a 410 or 411:

1. Brake not releasing. On vertical axes especially. The drive commands motion, the brake holds, error accumulates instantly.
2. Mechanical bind. Way lube failure, chip packing, a crashed axis, a failing bearing. Try moving the axis by hand with power off and the brake released.
3. Load beyond capability. A cut too aggressive for the axis, or acceleration parameters that no longer match a machine that has aged.
4. Parameters wrong or lost. After a battery failure or a botched restore, the in-position and excess-error parameters may not match the machine any more.
5. Drive not producing torque. Now the amplifier is a real suspect — particularly if the axis moves freely by hand, the brake is proven released, and the drive still cannot hold position.
6. Feedback fault. A pulse-coder or cable problem makes the CNC believe the axis is not moving when it is. This is the case where replacing a perfectly good amplifier fixes nothing.

Safety, briefly but seriously

Before any of the above involves opening a cabinet: isolate, lock out and verify. DC-bus capacitors in a drive section hold a lethal charge after the disconnect opens, and the manufacturer specifies a discharge time because that time is real. The control-of-hazardous-energy requirements (osha.gov) and the electrical practices for industrial machinery in NFPA 79 (nfpa.org) are both written in response to incidents on equipment exactly like this. Power-conversion equipment construction requirements come from the UL 508C lineage (ul.com), and the wider drive-system standards from the IEC 61800 series (iec.ch).

When to ship the amplifier

Send the module when the evidence points at it rather than past it:

• Every axis faults at once and the PSM is implicated — send the power supply module
• The amplifier shows a fault code on its own indicator and control power is confirmed present
• A fuse blows or the breaker trips when the drive section is energised
• A 411 persists on an axis that moves freely by hand with the brake proven released
• There is visible or olfactory evidence — discoloured components, a burnt smell, a bulged capacitor
• The fault followed a motor or cable short, which commonly takes the power stage with it

Send the amplifier only — no motors, no cables. Include the exact alarm number, the diagnostic bits, whatever the amplifier indicator showed, and what the machine was doing when it faulted. Include the machine make and model too; it lets us test against the right configuration.

If the evidence instead points at the motor, encoder or cabling, the free evaluation is where that gets settled without you spending anything. That decision is worked through generally in is it the amplifier or the motor?, and the module you are holding is decoded in the A06B part-number guide.

What the repair costs

As of September 2026, FANUC alpha, alpha i and beta servo amplifier repairs typically run $650–$1,500. Spindle amplifiers (SPM) run $750–$1,800 and power supply modules (PSM) $500–$1,200, both comfortably under OEM exchange pricing. A16B and A20B control boards, which is where some 401 chains actually terminate, run $300–$800. The bench evaluation is free, the quote is firm before any work, and the repair carries a 24-month warranty. Full context sits in servo amplifier repair cost.

Against that, OEM exchange amplifiers are quoted from $2,500 where the exchange programme still covers your generation at all — and for Series 0, 16, 18 and 21-era machines it frequently does not, which is precisely why component-level repair is the surviving service path for a large installed base.

Set either number against downtime and the arithmetic stops being close. ABB's Value of Reliability survey of 3,215 plant-level respondents put average unplanned downtime at $125,000 per hour globally; manufacturing productivity research (nist.gov) and federal producer price data (bls.gov) both point the same way, which is why rush service at $149 is usually the cheapest line on the invoice. When you are ready, start a repair and pack it per the shipping guide.

Frequently asked questions

What does FANUC alarm 401 mean?

It means the CNC enabled the servo system but the amplifier never returned its ready signal. The most common causes are a power supply module that did not come up, a main contactor that did not close, an open emergency-stop or overtravel circuit, missing control power at the amplifier, an interface cable fault, or a failed amplifier. If every axis alarms simultaneously, suspect the shared power supply module rather than the individual amplifiers.

Is alarm 410 or 411 the servo amplifier?

Usually not. Both are excess position error — the axis is not where the control commanded. Mechanical causes dominate: a brake that did not release, a binding axis, an overloaded cut, or parameters that no longer match the machine. The amplifier becomes the prime suspect only when the axis moves freely by hand with the brake proven released and the drive still cannot hold position.

What is the difference between alarm 410 and 411?

410 is excess position error while the axis is stopped; 411 is excess position error during motion. A 410 typically means the axis is being pushed or dragged out of position while it should be holding, and a 411 typically means the axis cannot follow the commanded move.

What should I do about alarm 414?

Treat it as a pointer, not a diagnosis. Alarm 414 indicates a digital servo fault whose detail lives in the diagnostic bits at DGN 200, 201 and 204. Read and write down those bits per axis before clearing the alarm — they identify overcurrent, overheat, feedback disconnect and pulse-coder faults specifically, and they are the difference between a targeted repair and a guess.

Do I need to send the motor along with the amplifier?

No — send the amplifier module only. Motors and cables are not needed for an amplifier evaluation and add significant freight cost. If our evaluation determines the fault actually lives in the motor, encoder or cabling, we tell you and you pay nothing.

How much does a FANUC servo amplifier repair cost?

Typical alpha, alpha i and beta amplifier repairs run $650–$1,500 depending on the failure and module size. Spindle amplifiers run $750–$1,800 and power supply modules $500–$1,200, against OEM exchange units quoted from $2,500. The evaluation is free and the quote is firm before any work begins.

Do you repair amplifiers for machines FANUC no longer supports?

Yes. Legacy Series 0, 16, 18 and 21-era amplifiers are a specialty, and for many of those generations third-party component-level repair is the only remaining service path. Donor components for those families remain available.