Hi
I assume you mean the habitation control panel fitted by the converter, i.e. nothing to do with Fiat.
Sometimes these used 7 segment numeric displays to show letters, in a crude sort of way with a mix of upper and lower case characters.
BU02 doesn't mean anything to me. Just wondering if it is actually displaying BU22 which might mean Buzz or Buzzer ?
If you can find out the maker of the control panel (e.g. Sargent) you should be able to source a user guide.