Service Case Overview | Recovery from FANUC Servo Alarm SV0366

FANUC Servo Alarm SV0366 (X-Axis Pulse Coder Error) Occurring at Power-On

• Servo alarm SV0366 appears immediately after power is turned on
• Machine cannot start or operate
• Often misdiagnosed as a servo motor failure
• Root cause: contamination of the X-axis linear scale
• Alarm resolved through cleaning and corrective parts handling

Customer & Machine Information

• Region: Chubu area, Japan
• Customer: Machine manufacturer (manufacturing industry)
• Machine: NC Screw Rotor Grinding Machine by Mizoguchi Kikai
• CNC: FANUC 31i-MB

Customer Inquiry

The customer contacted us regarding an NC screw rotor grinding machine that failed to start due to an alarm occurring at power-on. When the power was switched on, an X-axis pulse coder alarm appeared and could not be cleared. As a result, the machine was completely unable to operate.

Alarm details:

• Alarm code: SV0366
• Description: X-axis pulse miss

Because the machine was located in the Chubu area and production was affected, the customer requested urgent fault isolation and recovery.

Initial Assessment

Based on the alarm code and the information obtained during the initial hearing, we identified a potential abnormality in the X-axis servo feedback system. On FANUC 31i-MB controls, SV0366 (Pulse Miss) can be triggered by issues related to feedback signals, such as:

• Linear scales
• Feedback cables
• Signal transmission abnormalities

After arriving on site and checking the actual machine, we confirmed that no feedback signal was being output from the X-axis linear scale.

Symptoms and Impact of the Servo Alarm

Because the alarm occurred immediately at power-on:

• Servo ON was not possible
• The axis could not move at all
• Homing operation was impossible

At first glance, the symptoms strongly resembled a servo motor failure. However, the X-axis configuration of this machine did not use a servo motor, indicating that further investigation was required.

Root Cause Investigation and On-Site Work

Our engineers conducted a detailed inspection of the NC thread grinding machine.

Diagnostic Findings

• Confirmed SV0366 (X-axis Pulse Miss) on FANUC 31i-MB
• Visual inspection of the X-axis linear scale :Oil contamination observed
• Signal check : No output signal detected
• Internal inspection :Oil contamination found inside the scale

Based on these findings, we concluded that contamination of the X-axis linear scale caused a feedback signal failure, which triggered the servo alarm.

Corrective Action

We removed the X-axis linear scale and performed a thorough cleaning process. In addition, we inspected:

• The linear scale body
• Associated cables

Where necessary, parts replacement and corrective handling were carried out to ensure stable signal output. After completing the work, we re-powered the machine and verified alarm status.

Results and Customer Feedback

After the corrective work:

• SV0366 (X-axis Pulse Miss) was completely cleared
• No alarms occurred at power-on
• The machine started normally and returned to full operational status

The customer commented: “We assumed it was a servo motor issue, but your accurate fault isolation really helped us. Thank you.”

Experiencing Servo Alarms?

Servo alarms are not always caused by motor failures. If you are experiencing issues such as:

• Servo alarms occurring at power-on
• Axes that cannot move
• Homing operations that cannot be performed

Seibu Shoko provides on-site fault isolation and recovery support. Please feel free to contact us for assistance.