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Power down CNC and disconnect main power. Remove servo amplifier cover and visually inspect the inverter cooling fan for physical damage, debris accumulation, or bearing seizure. Attempt manual rotation to check bearing condition.

Allow 5 minutes after power-down for DC bus capacitors to discharge before touching amplifier components.

Navigate to DIAGNOSIS screen > SYSTEM > SERVO and check fan status bits. For αi/βi series amplifiers, observe diagnostic data showing fan rotation status (typically shows 0=stopped, 1=running).

Measure fan voltage at fan connector with power restored. Should read DC voltage per amplifier model (typically 24VDC or 48VDC). Verify against amplifier specification label on unit.

Replace cooling fan with FANUC OEM part number specific to amplifier model (e.g., A90L-0001-0511 series). Reset alarm via SYSTEM > ALARM > RESET after replacement, cycle power, and verify fan operation through diagnostic screen.

With power disconnected, check internal fuse F1 or F2 on amplifier control PCB (location varies by αi/βi series model). Reference amplifier maintenance manual schematic for exact fuse location and rating.

Verify main DC bus voltage is below 50VDC using insulated multimeter before internal inspection.

Measure DC voltage at fan connector J5 or J6 (model-dependent) with CNC powered on. Compare against specification: typically 24VDC ±10%. If voltage absent or low, suspect control PCB fan driver circuit failure.

Temporarily connect known-good replacement fan to verify power supply functionality. If replacement fan operates normally, original fan is defective. If replacement also fails, power supply circuit requires PCB-level repair or amplifier replacement.

If fan power supply circuit is confirmed failed, replace control PCB assembly or entire amplifier module per FANUC service recommendations. Clear alarm and monitor through DIAGNOSIS > SERVO for 30-minute continuous run test.

With CNC running and alarm present, perform visual confirmation that inverter fan is physically rotating. Use tachometer or strobe light to verify actual fan RPM is within specification (typically 2000-3000 RPM depending on model).

Check fan feedback connector (typically 3-wire: power, ground, tachometer signal) for loose pins, corrosion, or damaged wiring. Reseat connector and verify continuity of tachometer signal wire from fan to control PCB.

Using oscilloscope, measure tachometer output signal at control PCB input (typically 2-5VDC square wave proportional to RPM). If fan rotates but no signal present, replace fan assembly including integrated tachometer sensor.

If tachometer signal is present but alarm persists, suspect control PCB fan detection circuit failure. Replace control PCB or amplifier module. After replacement, verify operation via DIAGNOSIS screen and parameter 2111 (amplifier type verification).