Fanuc 401: Fanuc Series 0i Error Code 401

Symptoms

The CNC system displays alarm 401: SERVO ALARM: n-TH AXIS VRDY OFF. The servo amplifier READY signal (DRDY) for the specified axis has turned off, indicating the amplifier is no longer ready to drive the motor.

Possible Causes

Servo amplifier internal fault causing it to go to a not-ready state
Emergency stop circuit activated, cutting power to the amplifier
Servo amplifier power supply (24V control or 3-phase main) is lost
Connection cable between the CNC and servo amplifier is loose or damaged

Troubleshooting Steps

Step 1: Check if the emergency stop button is pressed or the ESTOP circuit is open
Release the E-stop and reset; verify the ESTOP chain is complete
Step 2: Verify the servo amplifier has both control power (24V) and main power (3-phase)
Measure voltage at CX1A (200V control) and the main input terminals
Step 3: Inspect the FSSB fiber optic cable or feedback cable between CNC and amplifier
Cable should be properly seated with no physical damage; the locking tabs should be engaged
Step 4: Check the servo amplifier status LED for any alarm code
A concurrent alarm code on the amplifier indicates the specific fault (e.g., LV for low voltage, OH for overheat)

How to Reset

To reset: 1) Fix the underlying cause (power, cable, amplifier fault). 2) Clear the alarm from the CNC (press RESET). 3) The servo should re-energize. Caution: If the amplifier has a hardware fault, it will not go ready and alarm 401 will recur immediately.

Common Replacement Parts

A06B-6240-B103: FANUC beta i SVM module (Match the exact model number from the nameplate; verify axis count and current rating)

Frequently Asked Questions

Q: What is VRDY?
VRDY (Velocity Ready) is the signal from the servo amplifier to the CNC indicating the amplifier is ready to drive the motor. When VRDY goes off, the CNC knows the amplifier is not in a ready state and triggers alarm 401.
Q: What is the difference between MCON and DRDY?
MCON is the CNC-to-amplifier signal commanding the amplifier to be ready. DRDY is the amplifier-to-CNC signal confirming it is ready. If MCON goes off but DRDY stays on (alarm 404), or if DRDY goes off without MCON command (alarm 401), there is a mismatch.

Fanuc Series 0i Error Code 401