Code 749: S–SPINDLE LSI ERROR
Issue Description & Safety Warning
The Fanuc 16i/18i/21i CNC has triggered a critical S–SPINDLE LSI ERROR alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.
Official Troubleshooting Steps
It is serial communication error while system is executing after power supply on. Following reasons can be considered. 1) Optical cable connection is fault or cable is not connected or cable is cut. 2) MAIN CPU board or option 2 board is fault. 3) Spindle amp. printed board is fault. If this alarm occurs when CNC power supply is turned on or when this alarm can not be cleared even if CNC is reset, turn off the power supply also turn off the power supply in spindle side.
Source: Fanuc Series 16i/18i/21i-B Operator's Manual, Appendix G Alarm List (B-63864EN/03)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Fanuc 16i/18i/21i CNC Alarms
Fanuc controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 749 on the 16i/18i/21i CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Fanuc's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 360 Fanuc 16i/18i/21i CNC codes.
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