If the above steps fail, the spindle amplifier itself may be faulty. Swap with a known working unit (ensure same model).

To prevent alarm 366, ensure:

If the alarm occurs primarily at high speeds, the internal photo-diode may be failing. Loose Connections

Broken, shorted, or loose encoder cable between the motor and the amplifier.

If you encounter this alarm, follow these diagnostic steps to isolate the fault: Common FANUC CNC Alarms List

For further technical support, you can consult professional repair resources like Tri Star CNC Services or the CNC Cookbook for a broader range of Fanuc fault codes. Fanuc Alarm Code List [ Definitive Guide ] - CNC Cookbook