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The CNC asked the axis amplifier to come online and the amplifier never returned its ready signal. This is a power alarm, not a program alarm: the axis control loop never even closes. Very often the real cause sits upstream, in the safety chain or in the cabinet supply, and not in the axis named on the screen.
For reference only: alarms change with the manufacturer, the control and the machine PLC. Check the full code in the official manual before doing anything. Legal notice.
Whenever it reappears after reset or there is an alarm in the amplifier, smell, heat or tripped fuses.
Try to reset only if the obvious safety cause is resolved; If it returns, stop diagnosis.
Confirm axis, servo alarm number and amplifier LED/code in Fanuc/OEM manual.
Choose the manual matching your control series, software version and machine builder. A support portal is a starting point; it does not establish applicability to every model.
A Fanuc axis drive lives in two worlds: the control side, where the CNC calculates positions, and the power side, where the power supply module and the amplifiers feed current to the motors. Before the CNC can command anything, the amplifier has to confirm it is fit to run: mains present and within tolerance, DC bus charged, no internal fault, and the safety enable circuit closed. That confirmation is the ready signal. The CNC demands it before energising the motor; if it asks and nothing comes back, or if it had the signal and loses it during operation, it cuts out and raises SV0401. Here is the key to not wasting time: the machine's safety chain, with its emergency stops, interlocked doors and power-circuit contactors, is exactly what feeds or inhibits that power side. So a pressed E-stop, a door whose contact does not make properly, a tripped thermal or a contactor that does not pull in give you precisely the same symptom as a failed amplifier. The fact that the alarm names one specific axis does not mean the problem is that axis either: amplifiers share a bus and a supply, and the first one in the chain to notice something missing is usually the one that complains.
In order, from the fastest and cheapest check to the most expensive.
01Walk the safety chain before touching anything else
Emergency stops at every station, including the one on the pendant and the one round the back that nobody looks at. Enclosure doors properly closed. Any loader or peripheral interlock. Reset whatever is pressed using the machine's normal procedure. This clears a large share of SV0401s and costs nothing. Never bypass an interlock to test.
02Note the axis and whether other alarms came up at the same time
If a single axis is named, write down which one. If several come up together, you already know it is not one motor: it is a common supply or the safety chain. If SV0401 appears alongside servo bus or power supply module alarms, those outrank it and have to be dealt with first.
03Power down and back up using the full procedure
A clean restart, following the machine builder's sequence and respecting the waiting time between off and on. If the fault was a supply transient or an incomplete bus charge, it goes away and stays away. If it comes straight back on power-up, it is a hard fault and it will not be solved from the operator panel.
04Ask what happened just before
A power cut, a storm, welding nearby, an axis crash, an amplifier swap or recent electrical work are all direct leads. An SV0401 that shows up the morning after work inside the cabinet almost always points at a connector left loose or a phase put back wrong.
05Hand the electrical diagnosis to maintenance
The status LEDs and displays on the amplifiers and the power supply module are the next useful piece of information, but reading them means opening the cabinet, and that is electrical maintenance work with the machine locked out, not an operator job. Same for fuses, control voltages and contactor states. Your part ends with a good description of the symptom: which axis, since when, whether it happens at power-up or during the job, and what was done beforehand.
Usually condensation, a slow DC bus charge or a contactor that pulls in late. Confirm it by noting whether it always clears after the same number of attempts and whether it is worse on Mondays or in cold weather. Do not let it slide just because it eventually starts: a contactor that is beginning to fail will fail completely in the middle of a part.
A voltage dip on the supply line. Confirm it by correlating the alarm times with what starts up in the shop; maintenance can log the incoming voltage. The fix is not in the machine, it is in the installation.
A worn or badly adjusted interlock contact on that door. It is confirmed by the fact that no other door triggers it. Maintenance adjusts or replaces it; it is never bypassed to keep producing.
Cabinet filter cleaning and thermal checks as part of the maintenance plan, because heat accelerates the ageing of contactors and capacitors. Power connector tightening at scheduled services, with the machine locked out. An orderly shutdown sequence at the end of the shift instead of dropping the main breaker while everything is running. And an alarm log: writing down the date and time of every SV0401 turns an intermittent fault nobody can explain into an obvious pattern within two weeks.
The axis does not receive ready status from the servo/amplifier or there is a power/drive failure. It is an alarm from the Servo / ejes area, and in the catalog it is listed as “Servo V-ready off”. On the machine you notice it like this: The servos do not enable, One or more axes stay locked, It may appear when starting the machine.
On the shop floor it almost always comes from one of these causes: Servo amplifier faulty, Drive power absent, Damaged motor/encoder cable, E-stop or safety circuit open. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Try to reset only if the obvious safety cause is resolved; If it returns, stop diagnosis. Resetting without fixing the cause first only hides the problem: SV0401 trips again and, in the meantime, you keep working with the machine in bad shape.
With the machine stopped and in a safe state, start with these checks: Check whether an e-stop button or a door/interlock is active, Note the affected axis and the amplifier alarms, Visually check connectors without opening energized cabinet. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With SV0401 active, avoid this: Do not bypass safety devices, Do not work on energised drives, Do not change servo parameters without backup. Skipping any of these points is what turns a minor stop into an expensive breakdown.
Whenever it reappears after reset or there is an alarm in the amplifier, smell, heat or tripped fuses. If you are not sure how far it goes, stop the machine and report it: a repeat alarm always works out cheaper than a crash.
It is critical: there is a real risk of damage to the machine, the tool or a person. Leave the machine stopped, do not keep resetting it to see if it goes away, and call maintenance before producing again. Confirm axis, servo alarm number and amplifier LED/code in Fanuc/OEM manual.
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