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How to Slove Fanuc Alarm 400?

JAN. 22, 2026

In Fanuc machine tools, 400 alarm is basically an “old friend”. Whether it is a machining center, lathe, or has been running for more than ten years of old equipment, many people sooner or later will encounter: 400 SERVO ALARM: n-th AXIS OVERLOAD!

Alarm a jump, the axis immediately stop, program interruption, sometimes reset can be a good while, sometimes a day can jump several times. Many of the site's first reaction is: “Is the servo bad?” But the actual processing more will find that 400 is not a simple point to the servo hardware damage alarm, it is more to tell you: this axis, now “work too much effort”.

Fanuc alarm 400 what is really saying?

From Fanuc's definition, the core meaning of 400 is just one sentence: the servo load on one axis has exceeded the system's allowable range.

The servo amplifier keeps an eye on the current. In the Fanuc system, current ≈ output torque ≈ load size. When the system realizes that the axis has been "pushing too hard” for an extended period of time, and it's not just a momentary spike, it triggers 400 and stops the axis. This is not a miscalculation, but a very typical protection logic. If it doesn't, the consequences can be overheating of the motor, damage to the amplifier power module, and even the screws and couplings being dragged down with it.

So the 400 was designed to save equipment, not to cause trouble.

Why is the 400 so common?

To be honest, most of the 400 alarms we've dealt with have been related to mechanical conditions, rather than electronics.

The most typical situation is that the machine tool has been used for many years, the lubrication status is slowly deteriorating, the guide rail, silk rod began to astringent; machining parameters are added little by little, the cutting load is getting heavier and heavier; servo in order to keep up with the instructions, can only be added to the current. The beginning of just a little higher current, the system can tolerate; a long time, Fanuc feel "wrong”, 400 out.

There are also some more hidden situations, such as: silk rod bearings began to deteriorate, but not to the extent of jamming; coupling has a slight eccentricity; or even the shield, drag chain in the foreign body, resulting in local resistance.

These problems, push by hand may be "barely able to move”, but for the servo, it is a real burden.

Could the motor and amplifier be the culprits?

Yes, but not as much as you might think.

On the motor side, if the bearings start to wear out and the resistance to rotation increases, the current will naturally go up without any change in the program, and 400 will easily appear.

On the amplifier side, the old alpha and beta series are the ones we see the most. Used for more than ten years, the power module aging, current detection circuit drift, in the same working conditions, it is more likely to determine the overload than the "young time”.

In this case, it is not uncommon to see a phenomenon: the machine is not significantly overloaded, but the 400 occurs more and more often.

Sometimes the encoder takes the blame

Although the Fanuc 400 is not an encoder alarm, if the feedback signal starts to become unstable, the servo system will constantly "correct the position”. Correction = adding force = adding current. Up to a certain point, the 400 will still jump. We have seen a lot of cases, the final root cause is just the encoder cable aging, plug oxidation, the results of the scene has been suspecting the screw, suspect the motor.

How to judge on site, so as not to repair more and more chaotic?

Our experience is that the first do not rush to touch the parameters, not to mention the first step on the replacement of large parts.

First look at the shaft, push up in the power-off state is not smooth; listen to any noise, feel the motor temperature is not abnormal; see if the lubrication is normal. If the mechanical state is obviously a problem, then basically do not have to suspect the electronic parts down.

If there is no mechanical problem, and then go to compare the same type of shaft, load, current performance difference; if possible, the motor or amplifier adjusted to see if the alarm will "follow the run”.

The 400 is a very logical alarm, and as long as you do it the right way, it won't hide the answer on purpose.

Repair, or replace?

If it's lubrication, resistance, or mild deterioration, it's often more cost-effective to repair than to replace; if the amplifier has been overloaded many times, and has repeated the 400, then continuing to hold on to the amplifier will make the problem bigger.

We in Songwei to Fanuc servo to do testing, will certainly run in Fanuc test bench load conditions, rather than just look at the power up without alarm. Because 400 this kind of problem, without load, it is easy to "false normal”.

Finally, to say a true

Fanuc Alarm 400, the essence is not "system broken”, but the equipment is to remind you that the axis has begun to eat. As long as you go along with the "load from where” this idea to check, rather than blindly suspect parameters, suspect the program, most of the 400, can be cleanly resolved.

If you're having trouble with the Fanuc Alarm 400, and you're not sure if it's a mechanical, servo, or amplifier problem, it's better to put the suspect part in a controlled environment rather than trying it on the machine. You are also very welcome to come to our professional team in Songwei.

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How to Slove Fanuc Alarm 400?

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