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Power OFF the CNC and visually inspect the serial pulse coder cable from the motor to the servo amplifier (αi or βi series). Check for cuts, pinches, cable routing near moving parts, and excessive bending near connectors.

Disconnect all power and wait 5 minutes for capacitor discharge before handling cables or connections.

Inspect encoder connector at motor end and amplifier CN2 connector. Look for bent pins, contamination, corrosion, or loose locking mechanisms. Disconnect and reconnect firmly ensuring proper seating and locking.

Use proper ESD precautions when handling encoder connectors. Do not force connectors or bend pins.

Check diagnostic screen DIAGNOSIS > SERVO > select affected axis and observe encoder pulse counts while manually rotating motor shaft (power OFF, axis released). Counts should increment/decrement smoothly without jumps or erratic behavior.

If cable damage is found, replace the entire encoder cable with FANUC specified part number. After replacement, verify alarm clears and check servo amplifier LED displays steady green. Monitor DIAGNOSIS > SERVO > ALARM HISTORY for recurring alarms.

Inspect motor encoder housing and cable entry point for signs of coolant ingress, oil contamination, or environmental damage. Check encoder breather hole (if equipped) is not blocked.

Access SYSTEM > PARAM and verify encoder-related parameters: P1815 (serial encoder type), P1825 (encoder output division), and P1826 (encoder serial number). Cross-reference with motor nameplate data to ensure correct configuration.

Do not modify parameters without recording original values. Incorrect encoder parameters can cause immediate servo alarms or position errors.

Test encoder signal quality using DIAGNOSIS > SERVO > LOAD METER to observe current load variations while jogging axis slowly. Erratic current spikes may indicate encoder signal dropouts causing phase detection errors.

If contamination or encoder failure is suspected, replace the pulse coder unit or entire motor assembly. After replacement, execute encoder calibration procedure and verify position feedback using DIAGNOSIS > SERVO > POSITION (FPOS vs APOS should match within tolerance).

Serial pulse coders require ID registration to the servo amplifier. Use SYSTEM > PMC > I/O LINK or dedicated setup procedure per maintenance manual.

Check servo amplifier LED indicators on the affected axis. Refer to amplifier module (αi: A06B-6140/6150 series, βi: A06B-6270/6290 series) for LED codes. Solid red or specific flash patterns indicate internal amplifier faults requiring module replacement.

Verify proper grounding of CNC cabinet, motor frame, and servo amplifier chassis. Measure resistance between motor ground and cabinet ground (should be <1 ohm). Check encoder cable shield continuity and verify shield is grounded only at amplifier end, not both ends.

Use isolated multimeter. Do not create ground loops by grounding shields at multiple points.

Test by temporarily swapping encoder cable with known good axis (if system has identical axes). If alarm follows the cable, cable is faulty. If alarm stays with original axis, suspect amplifier or motor encoder. Check MAINTENANCE > SERVO > ALARM LOG for pattern of occurrence.

Only swap cables between identical servo amplifier models with same encoder types. Mismatched configurations can cause immediate alarms.