When these diodes go bad, the indication is typically a "PFD Not Communicating" message on the MFD, and loss of engine data on the PFD, assuming both are properly configured. ![][1] [1]: https://hf-files-oregon.s3.amazonaws.com/hdpavidynesupport_kb_attachments/2019/02-06/046cd5b2-835e-4194-979b-a64042121e7e/...