Troubleshooting

CNC Alarm Troubleshooting Guide

You have an alarm on the control and a spindle sitting idle. This CNC alarm troubleshooting guide walks through the checks that clear most faults, the ones you should never attempt, and how to tell a real failure from a nuisance alarm.

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Alarm code readingServo and spindle faultsOverride and recovery

CNC alarm troubleshooting guide showing a FANUC alarm number on the machine control

On this page

Symptom, likely cause, and first move

Work down the table in order. The first column is what you see, the second is what it usually means, the third is what to do before touching anything else.

Symptom	Likely cause	First move
Servo alarm on one axis only	Axis overload or mechanical binding	Handwheel the axis with feed hold on
Spindle alarm at speed	Tool imbalance or spindle load too high	Drop to 25% rapid, check tool runout
Overtravel and limit alarm	Axis beyond soft limit or stuck switch	Switch to jog mode and back off slowly
ATC or tool change alarm	Chip jam or misaligned tool pocket	Clear chips, verify pocket number
Coolant or lubrication alarm	Low tank level or blocked flow sensor	Refill, then check sensor on the alarm page
Overheat alarm on drive	Fan clogged or ambient above 40 °C	Clean cabinet filter, check fan rotation
Door or safety interlock alarm	Switch not made or wiring open	Close door fully, then check the interlock LED
Battery or absolute position alarm	Encoder battery below 3.0 V	Replace battery with power on, then re-home

Clear the cause, not just the code

A reset that works three times and fails on the fourth is a warning, not a fix. Find the cause, log it, and keep the machine running.

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Reading the code

The first minute after an alarm

The alarm page usually tells you three things: the alarm number, the message text, and sometimes the axis or device that raised it. Note all three before you press reset. Once you clear the alarm, the message can disappear from the screen and you lose your best clue. On FANUC controls, a servo alarm such as 401 or 414 points to the drive or the axis; on Siemens, a 2000-series alarm points to a specific drive object.

Do not clear the alarm and restart the cycle straight away. That is how a small fault becomes a scrapped part or a bent tool. Stay in single block, keep the feed override low, and watch which axis moves first. A servo alarm that trips the moment the Z axis lifts is a different problem from one that trips during a rapid traverse.

The operator is your best sensor. Ask what the machine was doing, what sound it made, and whether it had been running long enough to get warm. A fault that appears after two hours of cutting points at heat and thermal growth. One that appears on the first tool of the shift points at a cold machine or a power issue.

1
Record the numberPhotograph the alarm page before reset.
2
Note the axisMost alarms name the axis or device that tripped.
3
Check the logAlarm history shows what happened just before.

Which system the alarm belongs to

Alarm codes differ between Fanuc, Siemens, Heidenhain, Mitsubishi, and Haas, but they fall into a small number of families. Servo and spindle alarms come from the drive. PLC and machine alarms come from the ladder logic that watches doors, levels, and switches. Program alarms come from the part program itself and clear when the code is fixed.

Mechanical problems often show up as electrical alarms. A tight way cover, a dry linear guide, or a chip packed under the table raises the axis load until the drive trips. Before you swap a servo amplifier, push the axis by hand with the drives off and feel for a tight spot. That single check saves a lot of unnecessary parts.

Overheat and battery alarms are the quiet ones. A cabinet fan that has stopped, a filter mat full of dust, or an encoder battery near 3.0 V will not stop the machine today. They will stop it at 2 a.m. on a deadline. Put both on a monthly check and the surprise goes away.

1
Drive familyServo, spindle, and overheat alarms.
2
Machine familyDoor, level, pressure, and interlock alarms.
3
Program familyTool offset, arc, and macro alarms.

Judgment

When to keep going and when to stop

Some alarms are nuisance trips. A door interlock that needs a firmer close, a coolant level sensor that reads low when the tank is half full, a chip sitting on a limit switch. Clear the cause, reset, and run. If the same alarm returns within one shift, it is not a nuisance and you should treat it as a real fault.

Other alarms mean stop. Spindle overload with a hot nose, a grinding noise from a ball screw, burning smell near a drive, or an intermittent earth-fault alarm all point to damage that gets worse with every cycle. Continuing to run risks the spindle, the part, and the operator.

The rule we use in our own shop: if the alarm repeats after two clean resets with the same conditions, the machine stops and a technician looks at it. No third reset on a fault we do not understand. That habit keeps a 15-year machine running and keeps fingers away from a spindle that might start.

1
NuisanceClears once, does not return in the shift.
2
Real faultReturns after two resets, same conditions.
3
Stop nowHeat, noise, smell, or earth fault.

Prevention

Checks that keep alarms away

Most alarms we see on the floor started with something small that was ignored. A filter mat that was not changed, a way lube line that was pinched, a tool holder that was dropped once. A ten-minute weekly walk-around catches more than any diagnostic screen.

Check the cabinet filters and fan rotation. Confirm the way lube pump cycles and the tank is above the low mark. Look at the coolant concentration and clear chips from the tank and the conveyor. Wipe the limit switches and check the door interlock makes cleanly. None of this needs a technician.

Keep a simple log per machine: alarm number, date, what was done. After three months the pattern shows itself. The same axis, the same tool, the same shift. That log is worth more than a service contract, because it tells you where to look before the machine stops again.

1
WeeklyFilters, fans, lube level, coolant strength.
2
MonthlySwitch cleaning, battery voltage, belt tension.
3
AlwaysLog every alarm with the action taken.

Procedure

Step by step: clearing a common alarm

Use this order for a servo, overtravel, or tool change alarm. Do not skip a step because it looks obvious.

1
Stop safelyPress feed hold, then stop. Do not press reset with the spindle turning. Wait for the spindle to reach zero rpm before you open the door.
2
Read the alarm pageWrite down the number, the message, and the axis or device. Photograph the screen if the control scrolls the message away.
3
Check the obviousDoor closed, air pressure above 0.5 MPa, coolant level above the low mark, no chip on a limit switch. Most trips end here.
4
Isolate the axisSwitch to jog or handwheel mode. Move the alarmed axis away from the limit at 10% rapid. Listen for binding or a grinding note.
5
Reset onceClear the alarm and re-home the machine at low rapid. Watch the load meter. If load stays near 100% on a free axis, stop and inspect the mechanical side.
6
Restart in single blockRun the first tool in single block at 25% feed and 25% rapid. Return to normal only after two clean cycles.
7
Log and move onRecord the alarm, the cause, and the fix. If the same alarm returns within one shift, stop and call a technician.

FAQs

Common questions

Can I clear a servo alarm and keep cutting?

If the alarm was caused by a chip on a switch or a door that was not fully closed, yes. Clear it, jog the axis, and watch the load meter.

If the same servo alarm returns within one shift, no. That pattern points to a drive, cable, or mechanical fault that will damage the axis if you keep running.

Why does my machine alarm only when it gets warm?

Thermal growth changes the load on an axis or spindle. A bearing that is just starting to fail runs free when cold and tightens as it heats.

Check the cabinet fan and filters first. If those are clean and the alarm still appears after about two hours, look at the spindle bearings and the way lube feed.

What does an overtravel or limit alarm mean?

The axis has moved past its soft limit, or a limit switch is stuck. Switch to jog mode and back the axis off slowly at low rapid.

If the axis will not move, the switch or its wiring is likely open. Check the switch state on the PLC or ladder page before you force anything.

How often should I replace the encoder battery?

Replace it before it drops below 3.0 V, and always with the machine power on so the absolute position is kept. A typical service life is one to two years, depending on temperature.

If the battery alarm has already tripped and the position is lost, you will need to re-home the machine and reset the work offsets.

Should I reset an alarm more than twice?

No. Two clean resets with the same conditions is the limit we use. After that, the machine stops until someone looks at it.

Repeated resetting can hide a growing fault and, in the worst case, restart a spindle or axis while someone is working near it.

When is it worth calling a technician instead of troubleshooting?

Call when you see heat, smoke, a burning smell, or a grinding noise, or when an earth-fault alarm appears. Those are not operator-level faults.

Also call when the same alarm returns after two resets and the mechanical checks are clean. At that point the problem is inside the drive or the control, and guessing costs more than the visit.

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