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Check the two-digit alarm code on the second spindle amplifier's LED display panel. Record this code (e.g., 10, 20, 30, etc.).

Navigate to DIAGNOSIS > SPINDLE (path varies: SYSTEM > DIAGNOSIS > spindle or MAINTENANCE > SPINDLE DIAGNOSIS) and check the spindle alarm code and status for the second spindle unit.

Reference the spindle amplifier manual for the specific two-digit code. Common codes: AL-10 (DC link overvoltage), AL-20 (overcurrent), AL-30 (overload), AL-40 (encoder error), AL-50 (thermal overload). Check for loose power cables, U/V/W motor connections, and encoder cable continuity.

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If thermal overload (AL-30 or AL-50), verify cooling fan operation on amplifier and spindle motor. Check spindle load parameters 4080-4089 (second spindle) for correct motor rated current and overload settings.

Inspect the second spindle encoder cable (typically JA7B on amplifier side) for physical damage, connector seating, and pin condition. Check for oil or coolant contamination at connectors.

Navigate to DIAGNOSIS > SPINDLE and observe the second spindle encoder feedback data (position, speed). No reading or erratic values indicate encoder or cable failure.

Verify encoder power supply: measure +5VDC at encoder connector pins (typically pin 1 and pin 3 on serial encoder). Low voltage indicates cable fault or amplifier power circuit failure.

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If encoder is functional but alarm persists, verify parameter 4020 (second spindle encoder type) matches installed encoder: serial Ai encoder, separate detector, or built-in sensor. Incorrect setting causes detection errors.

Power off the machine. Measure three-phase input voltage at the second spindle amplifier input terminals (R, S, T). Verify 200VAC or 400VAC class nominal, balanced within 2%. Check for loose connections or burnt contacts.

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Inspect the amplifier's DC bus voltage. With power on (if safe), measure DC link voltage across P and N terminals: expect approximately 280VDC (200V class) or 560VDC (400V class). Excessive voltage indicates regeneration resistor or discharge circuit failure.

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Check the regeneration discharge resistor and associated thermal switch (if equipped) for the second spindle. An open thermal switch or failed resistor causes overvoltage alarms during deceleration.

If sub-alarm indicates IPM (Intelligent Power Module) or gate driver fault, the amplifier requires board-level repair or replacement. Contact Fanuc service for amplifier exchange and parameter backup before replacement.