IBM 3270 Maintenance Manual page 187

1. The following will appear on your screen.
2. Press each key slowly and verify that each
block changes to a character.
It is not important at this time that the
character is the correct character.
DID EACH BLOCK CHANGE TO A
CHARACTER?
NO Replace the keyboard. See Section 5,
"Removal/Replacement and Adjustments."
YES
3-300-4 Maintenance Information