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## Alarm 401 Definition
Fanuc servo alarm 401 is "Servo ready signal off" — the servo amplifier is not ready to operate. This alarm prevents press motion and must be resolved before production can resume.
## Common Causes
### 1. Drive Power Supply Issue
Check the DC bus voltage on the servo amplifier. Normal range: 280-340V DC for 200V systems, 560-680V DC for 400V systems.
If DC bus is low or zero:
- Check input AC voltage at drive terminals
- Check main contactor operation
- Check DC bus capacitors (measure with multimeter)
### 2. Emergency Stop Circuit
Alarm 401 triggers when the E-stop circuit opens. Check:
- All E-stop buttons (pull to release)
- Safety door switches
- Light curtain status
- Two-hand control circuit
Use the Fanuc diagnostic screen (DGNOS) to check I/O status of the E-stop chain.
### 3. Servo Amplifier Fault
If DC bus is normal and E-stop is clear, the amplifier itself may have faulted. Check the amplifier LED status:
- Green steady: normal
- Red flashing: fault (count flashes for fault code)
- No LED: power supply failure
### 4. CNC-to-Drive Communication
Fanuc uses FSSB (Fanuc Serial Servo Bus) between CNC and drives. A cable fault or connector issue causes alarm 401.
Check FSSB cable connections at both CNC and drive ends.
## Reset Procedure
1. Identify and fix the root cause (do not just reset)
2. Press E-stop, then release
3. Press RESET on CNC operator panel
4. If alarm persists, cycle main power
## Escalation
If alarm 401 recurs within 10 strokes after reset, the drive or motor has an intermittent fault. Call Fanuc service.