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Power down the CNC completely and discharge residual power by waiting 5 minutes. Open the control cabinet and locate the main CPU board (typically labeled as CPU BOARD or MAIN BOARD with designation such as A20B-8100-series or A20B-8200-series depending on control model).

Always follow LOTO procedures and verify zero voltage before touching internal components.

Inspect the CPU board for physical damage, burn marks, corrosion, or swollen components around the DRAM chip area. Check all board-to-backplane connectors for proper seating and oxidation on contacts.

If visual inspection shows no obvious damage, reseat the main CPU board by removing and reinstalling into the backplane connector. Ensure proper seating with even pressure on both ends of the board edge connector.

Power up and observe if alarm clears. If alarm persists, the main CPU board requires replacement or factory repair service. Contact Fanuc service to order replacement board with matching control software version, or arrange board exchange/repair.

Backup all programs, parameters, and PMC ladder using SYSTEM > MAINTENANCE > BACKUP or memory card before replacing CPU board.

Access DIAGNOSIS screen by pressing SYSTEM > DIAGNOSIS, then navigate to the power supply voltage monitor page (typically DIAGNOSIS > PMM or STATUS depending on control series). Record all DC voltages: +5VDC, +24VDC, ±15VDC supply rails.

Verify +5VDC supply is within specification: 5.0VDC ±0.25V (4.75V to 5.25V) under normal operating load. Check for voltage ripple using oscilloscope on accessible test points on power supply unit (PSU) - ripple should be less than 100mV peak-to-peak.

Use isolated oscilloscope probe to prevent ground loops when measuring live circuits.

Inspect the main power supply unit for swollen or leaking electrolytic capacitors, particularly the output filter capacitors on +5V rail. These capacitors typically fail after 7-10 years of operation causing increased ripple voltage.

If voltage is out of specification or excessive ripple is detected, replace the power supply unit. For controls with modular PSU (A06B-6096-series, A06B-6110-series, A06B-6117-series), replace the defective PSU module. Verify voltages stabilize before returning to service.

Check control cabinet ambient temperature using DIAGNOSIS screen temperature monitors if available, or use infrared thermometer to measure CPU board temperature. Fanuc controls typically specify maximum ambient of 45°C (113°F) and cabinet internal should not exceed 55°C.

Inspect all cooling fans in control cabinet for proper operation. Check control unit cooling fan (typically mounted on rear of control), power supply cooling fan, and any auxiliary cabinet fans. Verify airflow direction matches cabinet design (typically bottom intake, top exhaust).

Clean or replace air filters if clogged with debris, dust, or oil mist. Remove any obstructions blocking ventilation slots on control unit chassis. Ensure minimum 100mm clearance around control cabinet for proper air circulation.

Power down control before cleaning internal components. Use dry compressed air or vacuum - never use wet cleaning methods on electronics.

If temperature issues are resolved but alarm persists after cool-down period (30+ minutes), the DRAM has likely suffered permanent heat damage. Proceed with CPU board replacement as thermal damage to memory ICs is irreversible.