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The reference return sequence did not complete as it should. The axis failed to capture its zero point, or it tried under conditions in which the control cannot guarantee the captured point is the right one. Until it is resolved, that axis's machine coordinates cannot be trusted.
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 correcting the cause and repeating a safe reference run.
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.
On a machine with dog-based homing, reference return is not simply driving into a stop. The axis moves towards the reference area until a detector, the deceleration dog or cam, changes state; at that moment the control decelerates and starts looking for the next marker on the encoder's reference signal, which repeats once per revolution. Machine zero is established exactly at that marker, not at the dog: the dog only says where to start looking. That division of labour is what gives the process its precision, because the encoder marker is repeatable, but it is also what makes it fragile. If the axis starts too close to the dog, if it arrives too fast, if the dog is loose or dirty and gives an irregular signal, or if the encoder marker never arrives, the sequence does not add up and the control cuts out. One detail explains many cases people experience as mysterious: if the dog position relative to the encoder marker sits right on the boundary, the axis sometimes captures one marker and sometimes the next, and the machine homes one screw revolution out, which can be a zero that is a few millimetres different. Sometimes that gives an alarm and sometimes it does not, and when it does not, it shows up as parts shifted with no explanation. On machines with absolute pulse coders the sequence is different, and a reference alarm usually means the absolute position was lost and has to be re-established by the builder's procedure.
In order, from the fastest and cheapest check to the most expensive.
01Identify the axis and check it has clear travel
Before repeating anything, make sure the axis can travel towards the reference area without meeting a part, a vice, the tailstock or the turret. Repeating homing blind into an obstruction is the usual way of turning an alarm into a crash.
02Move the axis away from the reference area and repeat
If the axis started too close to the dog, the sequence has no room to develop. Jog it a reasonable distance in the opposite direction and repeat the reference return by the normal procedure. If it works cleanly second time, that was it and there is no fault.
03Watch the behaviour during the sequence
See whether the axis decelerates on reaching the reference area or runs straight past without slowing. If it does not decelerate, the dog signal is not arriving. If it decelerates and then fails to find zero, the problem is the encoder marker or the distance between dog and marker. That distinction is worth a lot to whoever repairs it later.
04Check whether it is repeatable and whether zero always lands the same
Home the axis several times and compare the resulting machine coordinate by taking it to a known point. If it ends up slightly different each time or with a fixed jump, the dog-to-marker setting is on the boundary, and that is fixed by adjustment, not by repeating.
05Rule out recent causes
An impact near the dog, a clean-up that moved a detector, work on the axis, a battery change or a parameter restore. If the alarm started right after any of those, you have your main lead.
06Dog, detector or parameter adjustment: maintenance
Repositioning the dog, replacing a detector, checking the encoder reference signal or adjusting the sequence parameters is maintenance work, with the machine locked out for the mechanical parts and with a backup taken first for the parameter ones. After any adjustment, zero has to be verified against a known point before producing.
Not enough travel for the sequence to develop. It is confirmed because moving the axis away and repeating always works. It is not a fault, but it is worth teaching the manoeuvre to the whole shift so it is not experienced as a machine failure.
The dog position relative to the encoder reference marker is on the boundary and it picks one marker sometimes and the next one other times. Confirm it by homing several times and comparing against a known point. It needs adjustment by maintenance; ignoring it gives shifted parts with no alarm.
A detector moved, dirty or damaged, or a dog shifted. It is confirmed by the axis not decelerating as it enters the reference area. Maintenance checks it; do not force homing over and over in the meantime.
A power-up routine that is always the same: axes parked at a known position at the end of the shift and a full reference return at start-up. Protection and cleaning of detectors and dogs inside periodic maintenance, especially in areas with chips and coolant. Verification of machine zero against a known physical point after any work on an axis or an encoder. And a log of how often homing fails per axis: when an axis starts failing occasionally, there is time to fix it before it stops the machine.
Fanuc does not correctly complete the axis reference return. It is an alarm from the Sobrecarrera area, and in the catalog it is listed as “Abnormal reference return”. On the machine you notice it like this: Failure during homing, The axis does not capture the zero point, It may appear after battery/encoder/sensor.
On the shop floor it almost always comes from one of these causes: Reference sensor, Encoder or Z signal, Incorrect return sequence, Reference parameters. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset after correcting the cause and repeating a safe reference run. Resetting without fixing the cause first only hides the problem: PS090 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 for clear travel, Do not repeat homing if you approach the physical limit. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With PS090 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 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 a stop alarm: the cycle will not continue until you fix the cause. It does not always mean serious damage, but it leaves the machine where it is and you have to check it before running again. Guidance reference. Always confirm the full text in the exact machine/control manual.
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