trouble code 54

Fine to be on your zuki side.
May I ask a little question?
My little samurai complains about his really hardcore headache, which comes in interrupts of code 54.
Poor car!
Is somebody out there, who knows about the diagnostic trouble codes?
I´ve read it´s an error in the fifth switch circuit but hardly can figure out, where this parts reside and what´s their specific use.

Is there any literature available?

Thx for help!