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Power off the machine completely. Verify the motor nameplate and encoder model number on the affected axis motor.

Check parameter 1815 for the axis showing the alarm. Cross-reference with FANUC documentation to ensure the parameter value matches the physical encoder type (serial, pulse coder, absolute, etc.).

If mismatch found, correct parameter 1815 to the proper encoder type code. Also verify parameters 1816 (encoder serial number) and 1820 (encoder output pulses).

Changing encoder parameters requires parameter write enable (PWE). Document original values before changes.

Power cycle the system and perform a reference return. Monitor for alarm recurrence and verify axis movement is correct.

Confirm whether motor or encoder was recently replaced on the affected axis. Obtain the exact model number and specification of the new encoder assembly.

Access servo parameters and compare parameter 1815, 1816, and 1820 against the new encoder specifications from FANUC documentation or motor datasheet.

Update all encoder-related parameters to match the new hardware. For absolute encoders, clear and re-establish the reference position using parameter 1815#1 (absolute clear) if required.

For absolute encoders, clearing reference position will require full machine re-homing and potentially work coordinate re-establishment.

Perform FSSB connection check from diagnostics screen, then execute reference return and test axis movement under low feedrate to confirm proper operation.

Power down and inspect the encoder cable and connectors from motor to amplifier for physical damage, corrosion, or loose connections. Reseat all connections firmly.

Ensure proper ESD precautions when handling encoder cables and connectors.

Check FSSB diagnostic screens (diagnostic page for FSSB status) to verify communication status and encoder detection on the affected axis channel.

Measure encoder cable continuity and shield ground connection with a multimeter. Replace encoder cable if any conductors show open circuit or intermittent connection.

If cable and connections are verified good but alarm persists, the encoder internal PCB may be faulty. Replace the encoder assembly and configure parameters per new encoder specifications.

Encoder replacement requires motor disassembly on some models. Follow proper motor handling and reassembly procedures to maintain accuracy.