FANUC 401 Spindle Alarm

In the world of CNC machining, encountering a FANUC 401 Alarm can bring production to a grinding halt. This specific error is one of the most common—yet occasionally frustrating—faults found in FANUC Alpha and Beta series drive systems.

Specifically, the 401 alarm stands for “V-Ready Off.” In simpler terms, the CNC control is expecting the velocity control unit (the drive) to signal that it is “ready” to operate, but that signal is missing.

What Does the 401 Alarm Actually Mean?

The 401 alarm is a servo-related fault. When you power on a CNC machine, the Power Supply (PS) module sends power to the Spindle and Servo modules. Once these modules are energized and pass their internal checks, they send a “Ready” signal back to the CNC.

If the CNC does not receive this “Ready” signal, it assumes the drive is not prepared to move the motors safely and triggers the 401 alarm to prevent damage.

Common Symptoms

• The machine will not move in any axis (X, Y, or Z).
• The spindle will not rotate.
• The alarm usually appears immediately upon trying to engage the “Ready” state (releasing Emergency Stop).

The 401 alarm is often a “blanket” alarm. It tells you the drive isn’t ready, but it doesn’t always tell you why. You must look at the LED display on the actual drive modules inside the electrical cabinet.

• If the drive shows “01”, “09”, or “AL-XX”: The 401 is just a result of that specific drive failure.
• If the drive is blank: You likely have a power issue or a blown fuse.
• If the drive shows “–“: The drive is waiting for a signal from the power supply.

The Most Likely Culprits

The Emergency Stop Circuit

This is the most frequent cause. If an E-stop button is pressed, or if there is a break in the E-stop string (like a faulty limit switch or a broken wire), the drive will never get the signal to turn on.

• Solution: Verify all E-stop buttons are released and check the 24VDC supply to the E-stop circuit.

Faulty Power Supply Module (PSM)

In many FANUC systems, the Servo and Spindle modules rely on the Power Supply Module to tell them to wake up. if the PSM is showing an alarm (like a “04” or “01”), the Spindle/Servo modules will remain in a “not ready” state, triggering the 401.

• Solution: Troubleshoot the PSM first. If the PSM doesn’t show “00” (Ready), the 401 will persist.

Blown Fuses or Tripped Breakers

Check the internal fuses on the drive modules and the main circuit breakers for the machine. A blown fuse on the input power or the control power will prevent the “Ready” signal.

Communication Cables (FSSB)

FANUC uses a High-Speed Serial Bus (FSSB) to communicate between the CNC and the drives. If the fiber optic cable or the copper cable (depending on the generation) is damaged or unplugged, the CNC cannot “see” the drive.

• Solution: Inspect the optical cables for kinks or cracks. Ensure they are firmly seated in the “COP” ports.

If the basics don’t solve the issue, follow this technical path:

Checkpoint	Action	Result
Input Voltage	Measure R, S, T at the Power Supply.	Should be ~200-230V or 460V depending on model.
DC Bus	Measure the link between modules.	Should be approx. $1.414 \times \text{Input Voltage}$ (roughly 300VDC or 600VDC).
External 24V	Check the 24V power supply for the control circuit.	If 24V is low or missing, the relays won’t trigger.
MCC Contactors	Listen for a “clunk” when you reset the alarm.	If you don’t hear the magnetic contactor, the drive isn’t getting high voltage.

To resolve a FANUC 401 Spindle/Servo alarm, you must identify which component in the chain is broken.

1. Clear E-Stops and check limit switches.
2. Look at the hardware LEDs inside the cabinet to find the “Master” alarm.
3. Check the 24V control power and the FSSB communication cables.
4. Inspect the MCC (Magnetic Contactor) to ensure it is closing properly.

By following these steps, you can usually identify whether the problem is a simple external switch or a more serious internal component failure.