Fanuc Series 0i Error Code 404

Fanuc PLC & VFD Major Servo

Model: Series 0i | Code: 404

Applicable Systems
Series 0i Series 16/18/21 Alpha Beta

Symptoms

The CNC system displays alarm 404: SERVO ALARM: n-TH AXIS VRDY ON. This means the CNC sent a READY OFF signal (MCON off) to the servo amplifier, but the amplifier's READY signal (DRDY) is still on. Or, DRDY was on at power-up before MCON was commanded.

Possible Causes

  • Servo amplifier internal relay or control circuit stuck in the ready state
  • Servo interface module (board) in the CNC is faulty
  • Noise or interference on the MCON/DRDY signal line
  • Amplifier firmware issue causing DRDY to remain active

Troubleshooting Steps

  1. Step 1: Power off the entire machine and power back on
    If the alarm clears after power cycle, it was likely a transient state
  2. Step 2: Check the servo interface module connection in the CNC
    The module should be properly seated; reseat if possible
  3. Step 3: Inspect the signal cable between the CNC servo interface and the amplifier
    Cable should be intact, properly connected, and shielded
  4. Step 4: If the alarm persists, the servo amplifier or servo interface module is faulty
    Swap with a known-good amplifier or interface module to isolate the fault

How to Reset

To reset: 1) Power off the machine completely. 2) Wait 30 seconds. 3) Power back on. 4) If the alarm recurs, the amplifier or interface module needs replacement. Caution: Alarm 404 indicates a potentially dangerous state where the amplifier may energize the motor without CNC command.

Frequently Asked Questions

  • Q: Why is VRDY ON considered an alarm?

    If the CNC has not commanded the amplifier to be ready (MCON is off) but DRDY is on, the amplifier may unexpectedly energize the motor, creating a safety hazard. The CNC detects this mismatch and alarms to prevent uncommanded motor movement.

  • Q: Is alarm 404 dangerous?

    Yes. It means the servo amplifier may be in a ready state when it should not be. The motor could move without a command. Always treat this alarm seriously and power off the machine immediately.

Related Fanuc Fault Codes

Source: FANUC Series 0i-Mate Maintenance Manual, B-64115EN

Category: PLC & VFD