What causes FANUC alarm 301?

Reference return inhibit parameter is set — the control is configured to prevent re-homing after initial home. Absolute encoder machines do not require reference return but a G28 was commanded in restricted mode. Machine safety function preventing reference return in current state. Dual-check safety function active that restricts reference return.

How do I fix FANUC alarm 301?

1. Check if the machine uses absolute encoders that do not require homing — G28 may not be appropriate. 2. Review the reference return inhibit parameter settings for this machine. 3. Verify machine is in the correct operating state to allow reference return if it is needed. 4. Consult the machine builder documentation for reference return requirements.

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FANUC Controller

Program/G-code · Basic · 10-20 minutes

Alarm 301

Fanuc Alarm 301 - RESETTING OF REFERENCE RETURN IS INHIBITED

Alarm 301 indicates a communication failure between the servo amplifier and the absolute pulse coder (APC) on the specified axis. This is typically caused by damaged or disconnected pulse coder cables, faulty connectors, poor shielding/grounding, or a failed absolute encoder battery allowing loss of position data. The CNC cannot receive valid position feedback from the encoder.

FANUCProgram/G-codeBasic10-20 minutes

301

Fanuc Alarm 301 - RESETTING OF REFERENCE RETURN IS INHIBITED

RESETTING OF REFERENCE RETURN IS INHIBITED

Root Cause Summary

Alarm 301 indicates a communication failure between the servo amplifier and the absolute pulse coder (APC) on the specified axis. This is typically caused by damaged or disconnected pulse coder cables, faulty connectors, poor shielding/grounding, or a failed absolute encoder battery allowing loss of position data. The CNC cannot receive valid position feedback from the encoder.

Safety

KEEP POWER ON until battery replaced
Position will be lost if power off

Causes and Fixes

1
Power off the machine. Inspect the pulse coder cable (typically shielded 9-pin or encoder-specific connector) at both the motor encoder end and the amplifier JD1A/JD1B connector. Check for bent pins, contamination, or looseness.
Ensure LOCK/OFF on the power supply before touching connections
2
Disconnect and reconnect the pulse coder connector firmly. Check continuity of all signal lines using a multimeter with the cable disconnected from both ends. Verify shield continuity to ground.
3
If cable damage is suspected, temporarily swap the pulse coder cable with a known good axis (if available) to isolate cable vs. encoder failure. Replace cable if confirmed defective.
4
After reconnecting, power on and check DIAGNOSIS screen > SERVO > (axis number) for encoder communication status. Monitor diagnostic 300-303 (encoder serial data) for valid changing values during manual movement.

DIY Feasiblehigh confidence

Call a technician if:

·Servo amplifier internal circuit fault suspected
·Multiple axes showing simultaneous battery alarms
·Absolute position data corruption occurs

Prevention

Replace APC batteries every 5-7 years preventively
Monitor battery voltage monthly via diagnostic screens
Maintain clean, dry environment around encoders
Inspect encoder cables during routine maintenance

Common Mistakes

Powering system during battery replacement causing position loss
Using incorrect battery type or voltage rating
Ignoring intermittent voltage readings
Failing to check cable connections before battery replacement

Related Alarms

300 302 308 401

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