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Power OFF the CNC control completely. Open the control cabinet and visually inspect all cooling fans (typically located in rear panel or PSM/Power Supply Module section). Check for physical rotation resistance, bearing noise, or visible damage to fan blades.

Ensure LOCK-OUT/TAG-OUT procedures before opening cabinet. High voltage present even when control appears off.

Check for dust accumulation on fan blades, air intake filters, and heat sink fins. Clean all cooling paths using compressed air (max 30 PSI) or vacuum. Replace air filters if excessively clogged.

Power ON the control and observe fan rotation immediately during boot sequence. If fan does not spin or spins intermittently, measure DC voltage at fan connector (typically 24VDC for control unit fans). Expected voltage: 24VDC ±10%. If voltage present but fan inoperative, replace fan assembly.

After fan replacement or cleaning, navigate to SYSTEM > DIAGNOSIS > #0395-#0399 (thermal monitor diagnostics) to verify temperature readings return to normal range (typically 30-45°C for control unit). Reset alarm using RESET key and monitor for recurrence over 30 minutes continuous operation.

Power OFF control. Locate the fan power connector (typically on PSM or main control PCB, labeled FAN1/FAN2). Inspect connector pins for corrosion, bent pins, or burn marks. Check harness continuity using multimeter from PSM output to fan connector.

High voltage AC present on PSM input side. Work only on DC output side with power OFF.

With power ON, measure DC voltage at fan connector using multimeter. Expected: 24VDC ±2V. If voltage below 20VDC, trace back to PSM output. Check PSM LED indicators: Green LED solid = normal, Red LED or no LED = PSM fault. Consult PSM label for specific LED codes.

If PSM voltage output is low or absent, check AC input voltage to PSM (typically 200-230VAC single phase). Verify circuit breaker CB1 (control power) is closed and measure voltage at PSM input terminals L-N. If AC input correct but DC output low, replace PSM unit.

After PSM or wiring repair, power ON control and verify fan operates at full speed. Monitor DIAGNOSIS screen #0395 (control unit temperature) for 15 minutes. Temperature should stabilize below 50°C. Document repair and update maintenance log.

Navigate to SYSTEM > DIAGNOSIS > screen and verify actual temperature readings at #0395 (CPU temperature), #0396 (power supply temperature), and #0397 (ambient temperature). Normal operating range: 30-50°C. If readings show >80°C but cabinet feels cool to touch, suspect sensor fault.

Verify fan is operating normally (audible and visible rotation). Use non-contact thermometer to measure actual temperature at CPU heat sink and compare to DIAGNOSIS screen value. Deviation >20°C indicates sensor calibration error or circuit fault.

Power OFF control. Locate thermal sensor (small thermistor near CPU or on main PCB). Check connector seating and wiring for damage. Measure thermistor resistance at room temperature using multimeter (typical value: 10kΩ at 25°C). Replace sensor if resistance out of range or open circuit.

CPU board contains ESD-sensitive components. Use proper grounding strap when accessing.

If sensor and wiring test normal but alarm persists, suspect main PCB thermal detection circuit fault. Contact Fanuc technical support with control serial number and DIAGNOSIS screen values. Main CPU board replacement may be required (performed by authorized Fanuc technician).