Spent 9 hours chasing a ghost in a King Air and it turned out to be a loose cannon plug behind the MFD
Thought I had a bad ADC the whole time until I wiggled the harness on a hunch and the airspeed came back to life. What's your go to move when the boxes test fine but the plane still lies to you?
Wiggle every connector you can reach before you trust a single box, because half the time the plane is telling the truth and the wiring is lying. Had a Navajo that kept flagging the fuel gauge and it was just a chafed wire hiding behind a panel where nobody ever looks. Nine hours hurts but at least it was a plug and not a $15k black box swap.