IBM 6151 Maintenance And Service Manual page 180

Step 4
A procedure number of 1 indicates that the system unit failed with
an error code displayed in the two-digit display.
1. Set the power switch on the system unit to Off.
2. Remove the top cover. If needed see Section 4.
3. If present, remove the options from slots Band D. Then,
remove all of the adapters from the adapter slots (record the
slot numbers).
4. Set the power switch on the system unit to On and wait 2
minutes.
IS "23" DISPLAYED IN THE TWO-DIGIT DISPLAY?
NO One of the remaining boards or options is failing. The
order of most likely failure is:
1.
The processor board
2. The system board
3. If present, the memory option in slot C.
Using the list above, exchange any FRU not already
exchanged. To test each new FRU, set the power switch on
the system unit to On and wait 2 minutes for 23 to display
in the two-digit display. When 23 displays, the last FRU
exchanged is the failing FRU.
Go to "Step 28" on page 3-0060-30.
YES Continue on the next page.
3-0060-4 Service Manual