What is a servo press?

A servo press uses servo motors for precise force and position control during forming operations.

How do I calculate servo press tonnage?

Use our free tonnage calculator at servopress.club/tonnage-calculator.html

Fanuc Servo Alarm 401 on Press: Diagnosis and Fix

admin

## 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.

anna_controls

Alarm 401 is the catch-all 'not ready' alarm. Work through it systematically: check E-stop circuit, check main contactor, check DC bus voltage, check for other pending alarms. 90% of the time there's another alarm hiding underneath that's the real cause. Clear all alarms and see which one comes back first.

raj_maintenance

We get this after every power outage. The fix is always the same sequence: reset E-stop, turn on control power, wait 10 seconds for DC bus to charge, then reset the alarm. If it doesn't clear, check the 24V control power supply —ours has failed twice in 5 years and gives the same alarm.