Even 'proper' car mechanics spend a lot of their time swapping defective black boxes that the computerised diagnostic rig has told them to change, with little or no idea of how the boxes work.
Had an opel manta once that a mate crashed.
He fixed the car but the black box (or brain as mechanics refer to them) had a fault that upset the fuel injection so that the car would almost stall uponst being given full throttle.
Took it to three separate mechanics to get it fixed. (One specialist)
All said it was sealed and couldn't be fixed. New one was £400.
Took it apart meself and found two litle dials inside. Turned one of them and hey presto , fault fixed.