What causes FANUC alarm 405?

Machine was too close to the reference position when homing was initiated, causing the deceleration dog signal to be missed. The deceleration limit switch (dog switch) is faulty, misaligned, or not making proper contact during the homing move. Encoder feedback issue or loose encoder cable causing the servo to lose position during the return sequence. Grid shift parameter set incorrectly causing the zero point to fall outside the valid detection range. Servo drive fault or amplifier error interrupting the reference return sequence before completion.

How do I fix FANUC alarm 405?

1. Move the axis manually at least 100-200mm away from the reference position, then attempt the zero return again from that further starting point. 2. Inspect the deceleration dog and its limit switch for proper alignment, damage, or debris. Confirm the switch activates and releases cleanly as the axis passes over the dog. 3. Check all encoder cables and connectors on the affected axis for loose pins, damage, or intermittent connections. Reseat connectors at both the motor and drive end. 4. Check the servo amplifier for any additional alarm LEDs or fault codes that may indicate a drive-level fault contributing to the failed return. 5. Review parameters related to zero return such as the reference counter capacity (1821) and grid shift (1850) to confirm values are within valid range for the encoder being used. 6. If the alarm persists after all checks, perform a controlled power cycle and attempt homing again, monitoring axis movement for any hesitation or abnormal deceleration behavior.

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FANUC Controller

Servo/Axis · Intermediate · 20-45 minutes

Alarm 405

Fanuc Alarm 405 - SERVO ALARM: ZERO POINT RETURN FAULT

Alarm 405 occurs when the CNC detects an error during reference position return, typically due to incorrect detection of the grid/zero pulse from the position encoder, damaged encoder signal cables, incorrect parameter settings, or mechanical interference preventing proper deceleration zone completion. The servo system failed to establish the machine zero position using the reference point dog and encoder grid pulse sequence.

FANUCServo/AxisIntermediate20-45 minutes

405

Fanuc Alarm 405 - SERVO ALARM: ZERO POINT RETURN FAULT

Zero Return Fault

Root Cause Summary

Alarm 405 occurs when the CNC detects an error during reference position return, typically due to incorrect detection of the grid/zero pulse from the position encoder, damaged encoder signal cables, incorrect parameter settings, or mechanical interference preventing proper deceleration zone completion. The servo system failed to establish the machine zero position using the reference point dog and encoder grid pulse sequence.

Safety

Ensure machine is in EMERGENCY STOP before checking dog switch positions to prevent unexpected axis movement
Use proper lockout/tagout procedures when testing encoder connections with power applied
⚠️ EMERGENCY STOP required before checking dog switch positions
Be aware of potential pinch points when manually jogging the axis. Keep hands and clothing clear of moving parts.
Disconnect power to the machine before replacing any electrical components. Verify power is off with a multimeter.
High voltages are present in the servo amplifier. Only qualified personnel should work on the amplifier.
Ensure proper grounding of the machine and all electrical components to prevent electrical shock.
Use lockout/tagout procedures when testing encoder connections with power applied
Disconnect power before replacing electrical components and verify with multimeter
High voltages present in servo amplifier - qualified personnel only
Ensure proper grounding of machine and electrical components
Before working on any electrical components, ensure the machine is properly disconnected from the power source and locked out/tagged out. Verify the absence of voltage with a multimeter.
Be aware of the potential for stored energy in capacitors within the servo amplifier. Allow sufficient time for discharge after power is removed before touching any internal components.
If the axis is equipped with a counterbalance system (e.g., pneumatic or hydraulic), ensure it is properly supported before disconnecting any components to prevent uncontrolled movement.
Wear appropriate personal protective equipment (PPE), such as safety glasses and gloves, when working on the machine.
Ensure the work area is clean and well-lit to prevent accidents.
Never bypass or disable safety interlocks or emergency stop circuits.
Consult the machine's safety manual for specific safety procedures and warnings.
Allow time for capacitor discharge after power removal
Support counterbalanced axes before disconnecting components
Wear appropriate PPE and ensure clean, well-lit work area
Never bypass safety interlocks or emergency stop circuits
Before manually jogging the axis, ensure that the area is clear of any obstructions and that personnel are aware of the potential for unexpected axis movement.
When checking encoder connections, be aware that disconnecting the encoder cable while the servo is enabled can cause the axis to drift uncontrollably. Always disable the servo before disconnecting the encoder.
Be cautious when working near moving parts during manual jogging. Keep hands and clothing clear of the axis.
During reference return, the axis may move rapidly. Be prepared to press the EMERGENCY STOP button if necessary.
⚠️ Keep hands and clothing clear of moving parts during manual jogging
⚠️ Axis may move rapidly during reference return - be prepared to press EMERGENCY STOP
⚠️ Disable servo before disconnecting encoder to prevent axis drift

Causes and Fixes

1
Check DIAGNOSIS screen > Servo > DGN 300-399 for specific axis. Monitor DGN 300 (position deviation) and DGN 310 (grid signal detection). Verify grid signal count increments during manual movement.
2
Inspect encoder cable connectors at amplifier (JA7 for αi servo, CNP2 for βi servo) and motor encoder connector. Check for bent pins, corrosion, or loose connections. Measure encoder cable resistance between motor and amplifier.
Disconnect amplifier power (DC link discharged, wait 5 minutes) before inspecting connections.
3
For absolute encoders, check battery voltage at DIAGNOSIS > Absolute > Battery voltage display. Replace encoder battery if below 3.0V. Perform battery backup alarm reset and re-establish reference point using Parameter 1815#0 (APC).
4
If grid pulse still not detected, replace encoder cable first. If fault persists after cable replacement, replace motor encoder assembly and re-execute reference point return.

DIY Feasiblehigh confidence

Call a technician if:

·Encoder replacement required
·Servo amplifier showing hardware faults
·Mechanical repairs needed

Prevention

Regular encoder cable inspection
Periodic reference return accuracy checks
Maintain proper lubrication of axis components

Common Mistakes

Ignoring intermittent reference return issues
Adjusting reference position parameters without understanding machine geometry
Forcing reference return when mechanical binding exists

Related Alarms

401 410 414 417

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Always verify with your machine manual or a certified technician before performing service procedures.