This intermittent nature is common with the 366 alarm, and the troubleshooting journey usually follows these three chapters: The Vibration Culprit

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

Ensure the NC power and Servo power (AD converter) are not being interrupted abruptly, causing data loss.

By following a systematic troubleshooting approach—starting with the simplest checks (batteries, cables) before moving to more involved tests (motor swapping)—you can quickly pinpoint the root cause and make a cost-effective repair. For complex issues, especially those requiring encoder replacement, always consult your machine tool builder's documentation or contact a qualified FANUC service technician.

Have you recently performed any on that axis?

The encoder cable carrying position data from the motor to the drive can be damaged. Breaks, kinks, loose connectors, or oil/coolant contamination can severely degrade signals, triggering a pulse loss alarm.

To fix the problem, you must understand the mechanism. The Fanuc CNC operates on a fixed scan cycle—typically 4ms, 8ms, or 16ms. During this cycle, the CPU must:

: If the machine has multiple identical axes, swap the encoder cable or the servo amplifier with a known working one to isolate the fault. Replace Encoder/Motor

Troubleshooting Fanuc Alarm 366: SV Pulse Miss (Built-in) The (often displayed as SV0366 ) is a critical servo alarm indicating a "Pulse Miss" in the motor's built-in pulse coder. This occurs when the digital servo software detects that the encoder is losing data integrity—essentially "missing" pulses that are necessary to track the motor's exact position. What Causes Alarm 366?