Good description can be found here
http://www.iaa.ncku.edu.tw/board/upl...-present01.pdf
Boxes which have detected their own failure set 3 bits on the Sign Status Matrix The SSM of the ARINC 429 word which Indicates FW Failure warning of the Box
Boxes which are non operational have failures reported by downstream boxes ( lack of incoming 429 data would be an example)
Note If The recovery teams can get the QAR they will have far more data than is presented on the FDR - If AF had the QAR option