IBM Personal Computer AT Manual To Operations page 322

2
Did you receive a 6XX error?
XX:XX:XX
ERROR
SYSTEM UNIT 6XX
EN D - TEST 600
PRESS "ENTER" TO CONTINUE
?
(X can be any number.)
YES
Note the error and have your system unit
serviced. Be sure your Key Lock key is
with your system unit.
NO Go to Step 3.
600-2 Test