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This is the absolute pulse coder alarm family: low battery, absolute position data lost, pulse coder communication error, or the reference needing to be re-established. The specific number separates a simple battery warning from a position loss that has already happened, and the difference between the two is hours of work.
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 you have already lost absolute position or do not know the recovery procedure.
Reset after change/repair and reference confirmation.
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.
The absolute pulse coder holds two pieces of information: where it is within one revolution, read from its disc, and how many revolutions it has accumulated, held in a counter that is battery backed while the machine is off. That encoder also talks to the amplifier over a serial line using checked frames. The alarms in this group cover everything that can break in that assembly. If the battery voltage drops, the system warns while there is still time to change it. If the battery ran completely flat with the machine switched off, the revolution counter was lost and the absolute position is no longer valid: the control flags it and demands the reference be re-established before operating. If frames arrive with errors or stop arriving, the complaint is about communication and points at cable, connector or the encoder itself. And there is one situation that confuses a lot of people: after changing a battery, replacing an encoder or suffering a position loss, many machines keep showing the warning until the absolute reference re-establishment procedure is completed, which is not just pressing reset. That procedure is defined by the machine builder, not by Fanuc alone, and in many cases it means taking the axis to a known position and validating the value.
In order, from the fastest and cheapest check to the most expensive.
01Write down the exact number and the axis
Within this range not all the alarms mean the same thing. Look the specific number up in the control and builder manuals and put it in one of three categories: battery warning, absolute position loss, or encoder communication error. The rest of the diagnosis depends on which one you land in.
02If it is a battery warning, do not switch the machine off
With the warning active but the position still good, you have room to change the battery without losing anything, but every power-down is a risk. Leave the machine on until the change and get the exact battery the builder specifies.
03If it is a position loss, do not move in automatic
That axis's machine coordinate is no longer trustworthy, so any programmed move can go somewhere it should not. Lock out the automatic cycle and move on to re-establishing the reference using the builder's procedure, which is in the machine manual.
04If it is a communication error, treat the wiring as the prime suspect
Chafed encoder cable, coolant in a connector, loose shield. Check what is accessible from outside, without opening cabinets, and note whether the error always appears in the same part of the travel. The electrical inspection is a maintenance job.
05Take a backup before any intervention
Parameters and programs, stored off the machine and dated. Battery and absolute reference work is exactly where having a point to go back to pays off most.
06Verify positions before producing
With everything re-established, take the axis to a known position, check the machine coordinate matches, and run a check part if the job has tight tolerances. A badly re-established reference does not give an alarm: it gives out-of-tolerance parts, which is worse.
The common battery ran flat during the shutdown. It is confirmed because it affects every axis sharing that battery. It means a battery change and re-establishing the references by the builder's procedure, axis by axis.
That encoder's cable or connector, not the battery. It is confirmed by the repeatability of the position and by the other axes being clean. Wiring inspection by maintenance.
The absolute reference re-establishment procedure has not been completed, or the battery is the wrong type or badly seated. Confirm it by checking the installed type and by looking up the exact re-establishment procedure in the machine manual.
Battery changes by calendar, with a visible label on the machine showing the date of the last change. Condition checks before long shop shutdowns. Each machine's reference re-establishment procedure printed and kept beside it, because when you need it you need it right then. Encoder wiring protected and inspected at periodic maintenance. And a parameter and reference backup kept up to date, off the machine.
Fanuc reports communication errors, frame or low battery of the absolute pulse coder. It is an alarm from the Encoder / measurement area, and in the catalog it is listed as “APC/battery error press coder”. On the machine you notice it like this: APC Notice, It may lose the absolute reference, Alarm when turned on.
On the shop floor it almost always comes from one of these causes: Low battery, APC/encoder cable, Invalid pulse coder data, Incorrect battery change. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset after change/repair and reference confirmation. Resetting without fixing the cause first only hides the problem: APC300-APC306 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: Write down exact APC number, Back up the data if applicable, Follow OEM battery procedure. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With APC300-APC306 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.
If you have already lost absolute position or do not know the recovery procedure. 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 a warning: the machine carries on, but it is flagging something that will get worse if you let it go. Deal with it between parts, not at the end of the shift. Guidance reference. Always confirm the full text in the exact machine/control manual.
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