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Two different conditions related to the axis state: a servo ready signal that is not consistent with what the control expects, or a reference return that did not develop as it should. Both take the axis out of service and both usually point at the machine logic or the homing sequence rather than at the motor.
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.
If the axis does not home repeatably.
Reset after completing a correct reference.
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.
Between the control and the amplifier there is an exchange of status signals that has to follow an order: the control asks for enable, the amplifier confirms it is ready, and from there the axis moves. If that confirmation appears when it should not, or disappears when the control was expecting it, the consistency of the start-up breaks and the control flags it rather than carry on. That inconsistency can come from the amplifier itself, from the safety enable chain, or from the machine logic, which is what governs when the axes are enabled; that is why the cause is often in the PMC and not in the servo. The second condition has to do with reference return: the control watches that the sequence develops validly, with deceleration at the expected point and the reference marker captured where it should be. If the axis reaches the reference area at the wrong speed or in the wrong direction, if the deceleration signal does not appear, or if the sequence is interrupted, the result would not be a trustworthy zero and the control rejects it. The practical conclusion for the shop is that these two alarms are rarely solved by looking at the motor: they are solved by looking at the sequence, the signals and what was touched beforehand.
In order, from the fastest and cheapest check to the most expensive.
01Identify the axis and whether it is on enabling or on homing
If the alarm comes up at power-up or when trying to enable the axes, it is the servo ready side. If it comes up while the reference return is running, it is the homing sequence. Each leads somewhere different, so this separation comes first.
02Check the safety chain and the machine state
Emergency stops, doors, interlocks, the mode selected on the panel. Many enable conditions depend on the machine being in a specific state. Reset whatever is pressed by the normal procedure, bypassing nothing.
03If it is a reference problem, watch the axis behaviour
See whether it decelerates on entering the reference area or runs straight past, whether it is trying from a position that is too close, or whether it is going the wrong way. Move the axis away and repeat by the normal procedure with the travel clear: if it works second time, it was the starting position.
04Check for PMC or machine alarms in parallel
Many of these conditions originate in the builder's logic. If the control shows machine-specific messages alongside the servo alarm, those usually explain better what is missing: a pressure, a clamp, the position of some device.
05Repeat the power cycle once
A clean restart separates a transient from a hard fault. If after the restart the axis enables and homes correctly, log it as an incident and watch whether it repeats. If it comes back the same, stop retrying and collect data.
06Signals, dogs and reference parameters: maintenance
Checking the deceleration signal, the state of a detector, the enable logic or the reference sequence parameters is maintenance work, with the machine locked out for the mechanical side and with a backup taken first for the parameter side. Afterwards zero has to be verified against a known point.
That axis's reference detector or deceleration signal. It is confirmed because the axis does not slow down on entering the zone. Maintenance checks it; insisting on repeating the homing only adds crash risk.
A safety condition was open and the axes could not enable. It is confirmed because it goes away once everything is properly reset. If it repeats daily, there is an interlock or a contact worth checking before it fails completely.
Signal assignment or reference parameters altered. Confirm it against the machine's own backup. Maintenance corrects it by restoring the good copy and validating homing on every axis afterwards.
A start-up routine that is always the same, with the axes parked at a known position at the end of the shift and the homing order respected. Detectors and dogs kept clean and protected at periodic maintenance. Interlocks and door contacts checked as soon as they start giving occasional faults. A dated parameter backup belonging to each machine. And verification of zero against a known physical point after any work related to the reference.
Fanuc detects abnormal condition of servo ready or incorrect reference return on the indicated axis. It is an alarm from the Servo / ejes area, and in the catalog it is listed as “Servo ready ON or abnormal reference return”. On the machine you notice it like this: The axis does not become ready, Referencing failure, Alarm after power-up.
On the shop floor it almost always comes from one of these causes: Incorrect referencing sequence, Incoherent servo ready signal, Reference sensor, Servo/PMC parameters. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset after completing a correct reference. Resetting without fixing the cause first only hides the problem: SV0404/SV0405 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: Identify the axis, Check the travel and the reference dog, Compare with any additional PMC/servo alarms. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With SV0404/SV0405 active, avoid this: Do not repeat homing against the physical end, Do not modify servo parameters without backup. Skipping any of these points is what turns a minor stop into an expensive breakdown.
If the axis does not home repeatably. 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. Guidance reference. Always confirm the full text in the exact machine/control manual.
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