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The control has stopped receiving valid position feedback from that axis's pulse coder. It can be a physical disconnection, a damaged cable, contamination in the connector or a failure of the encoder itself. Until it is resolved, that axis's position cannot be trusted, and that is why the control will not enable the axis.
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, because it affects the position feedback.
Reset only after recovering encoder signal and confirming reference/position.
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 serial pulse coder does not send loose pulses: it sends continuous digital frames to the amplifier and the control, carrying position information and check bits. The receiver is not just expecting data, it is expecting correct data on time. If frames stop arriving, if they arrive with a check error, or if the encoder supply signals are not where they should be, the system concludes it has lost feedback and raises the disconnection alarm. It matters to understand why the control is so strict: the whole position loop closes on that data. If it kept moving the axis on doubtful feedback, it would be driving an axis blind into stops, parts and tooling. So this is not a negotiable alarm: until the signal is good, the axis does not move in automatic. The causes split between the mechanics of the wiring and the electronics of the encoder, and experience says the wiring wins by a mile: chafing in the cable carrier, a connector working loose from vibration, coolant getting in through a badly sealed gland and reaching the contacts, or a loose shield letting shop noise corrupt the frames. Encoder failure itself does happen, but it usually arrives after years, or following an impact or a coolant ingress.
In order, from the fastest and cheapest check to the most expensive.
01Identify the axis and do not move it in automatic
Note which axis the alarm names. Until feedback is recovered, the displayed position may not match reality. Do not run cycles or programmed moves on that axis, and bear in mind that if you move the axis by hand while the system is lost, the position will still be wrong when it comes back.
02Reconstruct the context: when, and during what
At power-up, during one particular move, always in the same part of the travel, after a wash-down with a hose, after a tool change that hit something. If it always appears when the axis is in one specific part of its travel, you have a cable chafing or a connector moving, and you know where to look.
03Look at what is accessible, without opening cabinets
With the machine stopped and safe, check what is in plain sight: motor connectors reachable from outside, cable carriers, cable glands, signs of coolant where there should be none. Do not open the electrical cabinet and do not unplug encoder connectors: that is for maintenance, with the machine locked out.
04Power down and up once, by the procedure
A clean restart separates a hard fault from an intermittent one. If the alarm comes straight back at power-up with nothing having moved, it is hard. If it clears and returns hours later, it is intermittent, and intermittents are nearly always cable or connector.
05Check whether other axes or other alarms are involved
If there are also servo bus alarms, or several axes complain at once, do not go looking at one encoder: look first at the common communication and the supply. If it is a single axis and everything else is perfect, the problem is in that branch.
06Hand it to maintenance with the history
Affected axis, when it appears, at what point in the travel, what was done beforehand, and whether it is hard or intermittent. Checking continuity and shielding, replacing cable or encoder, and validating the reference afterwards is electrical work. After an encoder is replaced, that axis's reference has to be re-established using the builder's procedure before producing.
An encoder cable chafed or pulled tight at that point in the cable carrier. It is confirmed by the repeatability of the position. It needs the cable replaced or rerouted, which is maintenance work; continuing to move the axis only makes the damage worse.
Coolant or water got into a connector. It is confirmed by the timing and by moisture in the area. Maintenance cleans and dries it, and the sealing has to be checked so it does not happen again.
Electrical noise or a loose shield. It is confirmed if it coincides with heavy equipment starting, welding nearby or a new drive installed in the shop. Maintenance checks it, looking at earthing and cable shields.
Inspection of cable carriers and their cables at periodic maintenance, looking for chafing and forced bend radii. Sealing of glands and connectors at the head and at the motors, especially on machines that use a lot of coolant. Cleaning at sensible pressure and without aiming jets straight at connectors and motors. Earthing and shields reviewed whenever new equipment is installed in the shop. And log every occurrence with time and position: on intermittent faults, that log is what finds the cable.
Fanuc detects position detection failure or disconnection of the encoder/pulse coder from the axis. It is an alarm from the Encoder / measurement area, and in the catalog it is listed as “Disconnection of pulse coder / encoder”. On the machine you notice it like this: The axis does not enable, Alarm when moving or starting, May appear with disconnection subcode.
On the shop floor it almost always comes from one of these causes: Encoder cable disconnected, Damaged press coder, Contaminated connector, Amplifier with a feedback fault. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset only after recovering encoder signal and confirming reference/position. Resetting without fixing the cause first only hides the problem: SV0416 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: Note axis and related diagnosis, Check accessible connectors with a secure machine, Do not move the shaft if the position is not reliable. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With SV0416 active, avoid this: Do not bypass safety devices, Do not manipulate energized electrical cabinet, Do not continue producing if the alarm reappears. Skipping any of these points is what turns a minor stop into an expensive breakdown.
Whenever it reappears, because it affects the position feedback. 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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