Sun Microsystems 220R - Manual (Page 213)

To indicate the beginning of POST diagnostics, the four LEDs briefly light all at once.
The monitor screen remains blank, and the Caps Lock LED flashes for the duration
of the testing.
If the system passes all POST diagnostic tests, all four LEDs light again and then go
off. Once the system banner appears on the monitor screen, the keyboard LEDs
assume their normal functions and should no longer be interpreted as diagnostic
error indicators.
If the system fails any test, one or more LEDs will light to form an error code that
indicates the nature of the problem.
Note - The LED error code may be lit continuously, or for just a few seconds, so it is
important to observe the LEDs closely while POST is running.
The following table provides error code definitions.
Caps Lock Compose Scroll Lock Num Lock Failing FRU
On Off Off Off Main logic board
Off On Off Off CPU module 0
Off On On Off CPU module 1
On Off Off On No memory detected
Diagnostics and Troubleshooting 213