10 Fanuc Alarms Every Machinist Will See (And How to Fix Them)

If you run Fanuc controls, you will hit these alarms. Not maybe. Will. Whether you're running a brand new Robodrill or a 90s-era 0i-Mate that's seen better days, these 10 alarms account for the vast majority of unplanned stops on the shop floor.

This isn't a manual reprint. No theory, no Fanuc documentation copy-paste. Just what actually happens, why it happens, and exactly what to do about it. Written by Axis Intelligence and verified accurate by a field service engineer with 10 years on the floor.

1. SV0401: Servo Amplifier Voltage Ready Off

What it means: The servo amplifier isn't getting stable DC bus voltage. The CNC is telling you it can't move axes because the power feeding the servo drives is missing or unstable.

Why you're seeing it:

• Power supply module failure or degradation inside the servo amplifier rack
• Incoming power problems: voltage sag, brownout, or a bad contactor not fully closing
• Blown fuse on the DC link or a failed capacitor bank that can't hold bus voltage

What to do:

1. Check the DC bus voltage at the servo amplifier. You should see around 300VDC on 230V systems, ~650VDC on 480V systems. Check your machine nameplate. If it's low or fluctuating, you have a power supply problem.
2. Inspect the magnetic contactor (MCC). Listen for it clicking when you power up. If it chatters or doesn't pull in, replace it.
3. Look at the charge lamp on the power supply module. If it's not lit or blinking abnormally, the PSM may need replacement.
4. Check incoming facility power. A bad leg on your 3-phase can cause this.

Full troubleshooting guide for SV0401 →

2. SV0410: Servo Position Error Exceeded

What it means: The difference between where the CNC commanded the axis to be and where the encoder says it actually is has exceeded the allowed threshold. The machine stopped itself to prevent a crash or damage.

Why you're seeing it:

• Mechanical binding. Worn ways, a damaged ballscrew, chips packed in the way covers, or a crashed axis that bent something
• Loose coupling between the motor and ballscrew. If the coupling slips, the motor turns but the axis doesn't move proportionally
• Servo tuning parameters got corrupted or were never right after a drive replacement

What to do:

1. Try jogging the axis slowly in both directions. If it moves freely one way but binds the other, you have a mechanical problem.
2. Check the ballscrew coupling. Grab the ballscrew and try to rotate it by hand (power off). Any play means the coupling is shot.
3. Look at diagnostic screen for the position error value. If it's climbing steadily during a move, the axis is fighting something mechanical.
4. If this started after a drive or motor swap, the servo parameters probably need adjustment. Check CMx (flexible feed gear) and velocity loop gain.

Full troubleshooting guide for SV0410 →

3. Alarm 300: Axis Needs Reference Return (ZRN)

What it means: The machine lost track of where its axes are. The absolute position data stored in the encoder backup memory is gone, and the control needs you to home the machine before it will let you run anything.

Why you're seeing it:

• The encoder backup battery died while the machine was powered off
• Someone swapped a servo amplifier or motor without doing the proper absolute position setup
• A power surge or hard crash corrupted the position data

What to do:

1. Manually reference return each axis. On most Fanuc controls: go to REF mode, move each axis to the reference point in the correct order (homing sequence depends on your machine builder's configuration — check the manual. Common practice is Z first to clear the work area, then X and Y).
2. If this keeps happening, replace the encoder backup battery. It's usually on the servo amplifier (A98L-0031-0012 or equivalent). Replace it with the machine powered ON to avoid losing position data again.
3. If the alarm persists after homing, the encoder itself may have a fault. Check for APC alarms (301-309) that follow.

Full troubleshooting guide for Alarm 300 →

4. PS0091: Reference Point Return Required

What it means: You tried to run a program or MDI command that requires the machine to know its absolute position, but it hasn't been homed yet. Similar to Alarm 300, but triggered by a specific action rather than at power-up.

Why you're seeing it:

• Machine was powered up but nobody ran the reference return sequence before trying to cut
• Encoder battery failure during the last power-off cycle
• An E-stop during a reference return interrupted the sequence, leaving one or more axes unhomed

What to do:

1. Go to REF mode and home all axes. It takes 30 seconds. Do it every time you power up.
2. If you get this alarm repeatedly even after homing, check the ZRN complete signals in the PMC diagnostic screen. One axis may not be reaching its reference switch.
3. On machines with absolute encoders (most modern Fanuc systems), this alarm after a normal power cycle means the battery is dead or dying. Replace it now before you lose position data mid-shift.

Full troubleshooting guide for PS0091 →

5. Alarm 503/504: Axis Overtravel

What it means: An axis hit its travel limit. Either the physical limit switch tripped (hardware overtravel) or the programmed soft limit was exceeded (software overtravel).

Why you're seeing it:

• A program tried to move past the machine's physical travel range
• Work offset or tool length offset is wrong, shifting the entire toolpath beyond limits
• After a crash or power loss, the machine's position reference is off, and what it thinks is safe travel actually isn't

What to do:

1. Don't panic. Press and hold the overtravel release button on the operator panel while jogging the axis back toward center. The button is usually labeled "OT RELEASE."
2. Check your work offset (G54, etc.) and tool length offset. A typo here is the most common cause.
3. If this happened during a reference return, the deceleration dog or limit switch may be out of position. Inspect the physical switch.
4. Review parameter 1320/1321 (software travel limits). If someone changed these, the allowed travel range may be wrong.

Full troubleshooting guide for Alarm 503 → | Alarm 504 →

6. Alarm 301: Encoder Battery Voltage Low

What it means: The lithium battery that keeps the absolute encoder memory alive during power-off is dying. This is a warning. If you ignore it, you'll get Alarm 300 next time you power down, and you'll have to re-home every axis.

Why you're seeing it:

• The battery is old. They last 2-5 years depending on how often the machine is powered off. Machines that sit unpowered over weekends or holidays drain batteries faster.
• Someone installed the wrong battery type during a previous replacement
• The battery connector is corroded or making intermittent contact

What to do:

1. Replace the battery NOW, while the machine is still powered on. This is critical. If you power off first, you lose the position data.
2. The battery is on the servo amplifier, usually accessible from the front. It's typically an A98L-0031-0012 (Fanuc part number) or equivalent 3V lithium cell.
3. Swap it, clear the alarm, and move on. Total job time: 5 minutes.
4. Buy a few spares and keep them in the maintenance crib. Write the install date on the battery with a marker.

Full troubleshooting guide for Alarm 301 →

7. Alarm 700: System Overheat Warning

What it means: The CNC control unit's internal temperature has exceeded its safe operating threshold (usually 55-60°C). The control is protecting itself from heat damage.

Why you're seeing it:

• The air filters on the control cabinet are clogged with shop dust, coolant mist, and grinding swarf
• A cooling fan inside the cabinet has failed or is running at reduced speed
• It's summer and the shop doesn't have adequate climate control near the machine

What to do:

1. Check the cabinet air filters immediately. Pull them out and clean or replace them. This often fixes it.
2. Open the cabinet and check that all fans are spinning. Failed fans are cheap to replace.
3. If the shop ambient temperature is consistently above 95°F near the machine, you may need a cabinet cooler (Rittal, Pfannenberg, etc.) or at minimum redirect some AC airflow toward the control.
4. Inspect the heat exchanger on the back or side of the cabinet. If it's coated in grime, clean it.

Full troubleshooting guide for Alarm 700 →

8. Alarm 414: Digital Servo System Error

What it means: Communication between the CNC and the servo amplifier over the FSSB (Fanuc Serial Servo Bus) fiber optic network has failed. The control can't talk to the drives.

Why you're seeing it:

• A fiber optic cable (FSSB cable) is damaged, kinked, or has a dirty connector
• The servo amplifier has an internal hardware failure
• An encoder feedback cable is damaged or has a corroded connector
• Electrical noise from a nearby VFD or welding operation is causing interference

What to do:

1. Check the FSSB fiber optic cables. Look for kinks, tight bends, or damage. Clean the connectors with a lint-free wipe.
2. Look at the LED indicators on the servo amplifier. If the ready light isn't green, the amp may be faulted. Note any blinking patterns and check the Fanuc servo amplifier manual for the specific code.
3. Swap the FSSB cable if you have a spare. This is the cheapest and most common fix.
4. If the problem started after nearby equipment was installed, check for electrical noise. Proper grounding and cable routing matters.

Full troubleshooting guide for Alarm 414 →

9. Alarm 010: Improper G-Code

What it means: The program contains a G-code that the control doesn't recognize or that isn't valid in the current context.

Why you're seeing it:

• A typo in the program. G1000 instead of G100. G96 on a mill that doesn't have constant surface speed.
• The G-code is valid but the machine option isn't purchased. G68.2 (5-axis tilted work plane) requires the 5-axis option. G12.1 (polar coordinate interpolation) requires that option. Fanuc sells these separately.
• A program was posted for a different control type (Haas, Siemens) and has codes that don't exist on Fanuc

What to do:

1. Look at the program line the alarm stopped on. The block number is shown on the alarm screen.
2. Check if the G-code is valid for your specific Fanuc series and options. Not all G-codes work on all models.
3. If you're using a CAM post processor, make sure it's configured for the correct Fanuc model. A generic "Fanuc" post will cause problems.
4. For option-locked G-codes, you can sometimes work around them with alternative programming methods, but the real fix is purchasing the option if you need it.

Full troubleshooting guide for Alarm 010 →

10. Alarm 920: Servo Ready Off

What it means: The servo system isn't ready. This is a broad alarm that means the drives aren't in a state where they can move axes. It's often a secondary alarm that shows up alongside a more specific one.

Note: Alarm 920 usually appears alongside a more specific alarm (like SV0401). Fix the primary alarm first — 920 will clear once the root cause is resolved.

Why you're seeing it:

• Emergency stop is active (most common). Someone bumped the E-stop or there's a door interlock issue.
• The main contactor isn't closing. The relay that connects power to the servo drives is stuck open.
• DC bus voltage isn't charging. Related to SV0401 but at an earlier stage.
• A safety circuit is open (door switch, light curtain, safety relay)

What to do:

1. Check the obvious first: is the E-stop released? Is the machine door closed? Reset the E-stop and try again.
2. Listen for the main contactor click when you turn on the servo drives (usually by pressing "Servo On" or releasing E-stop). No click = contactor problem.
3. Check the safety relay chain. If your machine has door interlocks or light curtains, a fault in any of those will hold off servo ready.
4. Look at the PMC diagnostic screen for the servo ready input signal. If it's off, trace the safety chain backward to find what's holding it.

Full troubleshooting guide for Alarm 920 →

The Bottom Line

These 10 alarms will account for the majority of your Fanuc-related stops. Some are 5-minute fixes (battery swap, filter cleaning, E-stop reset). Others point to real mechanical or electrical problems that need proper diagnosis.

The key is knowing which is which. A tech who understands what SV0401 actually means can have the machine running in 20 minutes. A tech who doesn't will spend hours guessing.

Every alarm code in this list links to a detailed troubleshooting page in our alarm code database. It's free, it's searchable, and it covers Fanuc, Haas, Siemens, Heidenhain, Mazak, Okuma, and more. Over 3,500 codes with real diagnostic steps written by people who've actually fixed these machines.

Bookmark it. You'll need it at 2am.