Lenovo ThinkStation 2551 Hardware Maintenance Manual page 76

d. Extended video memory
e. External Cache
f. External Cache RAM
g. Hard disk drive
h. Diskette drive
3. Turn on the computer to re-test the system.
4. Repeat steps 1 through 3 until you find the failing device or adapter.
If all devices and adapters have been removed, and the problem continues, replace the system board.
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ThinkStation Hardware Maintenance Manual