After nine years of this the pattern is boring and useful. Three faults account for most of what comes in.
Earth straps. A car earths through braided straps and bolted contacts, and salt air turns a bolted contact into a resistor. The symptoms are baffling and unrelated to each other: dim lights, a fan that runs slowly, a radio that resets when you indicate, a starter that turns lazily. Cleaning two bolts fixes all of it at once, and it is the cheapest genuine repair in this workshop.
Connectors with green powder in them. Multiway plugs down low on the car, in the wheel arches and under the battery tray, fill with corrosion and then the circuit works when it feels like it. They are found by looking, one at a time, which is an hour of somebody's life and cannot be shortened.
Relays that fail hot. Perfect in the morning, useless at four in the afternoon, and they test fine on a bench because a bench is not thirty eight degrees.
None of those three is a computer, and all three get blamed on one.