IBM 5150/5155/5160 - Power On Self Test (POST) - Stuck Key
You are here because your IBM 5150/5155/5160 is displaying a 'xx 301' error on power up, where xx is a byte.
The presence of the byte before the 301 is the POST indicating that a key on the keyboard is stuck down (as reported by the keyboard). The byte is an indicator as to which key.
IMPORTANT: Note too that some other problem in the keyboard may be causing the keyboard to report a stuck key (even though there is no stuck key).
IMPORTANT: Note that more than one key could be stuck down, in which case, what is reported is the first stuck key.
Identifying the stuck key
STEP 1: Convert the byte located before the 301 from hexidecimal to decimal.
STEP 2: Find the IBM Technical Reference for your computer. In it, of the many different keyboard layout diagrams, locate the one that matches your keyboard. The diagram has numbers next to each key.
STEP 3: Looking at the diagram, match the decimal number from step 1 with a key.
Identification example
On my IBM 5160, I see "24 301".
I convert the 24 from hexidecimal to decimal, which results in 36.
I go to the IBM 5160 Technical Reference, and within that, I locate the keyboard layout diagram that matches my (repeat: my) keyboard. It is here.
Looking at the diagram for my keyboard, I match 36 with the J key.
'0E 301'
See here.
Rectification
Almost every time that I have fully disassembled an IBM 5150/5155/5160 keyboard, then put it back together, I have encountered a 'xx 301' error at power up time.
Often, the fix was simply to remove the cap of the offending key (using a screwdriver) then putting the key cap back. Once, I had to dismantle and reassemble the keyboard again.
Of course, that is not always the fix.