[aerohead]

Allegedly, two capacitors in the T-100's mother board ultimately gave up the ghost, leaking acid, and eating away two traces of the printed circuit board.
The computer was sent off for repair. When it was returned and installed, the truck failed to start.
Presuming the computer was 'fixed' I went on a month's spree, chasing through the entire engine management system before coming to the realization that, the computer had not been repaired at all.
After sending the computer back, I got a call from the technician, apologizing for having missed two additional dead caps, and dissolved circuit board traces.
I got the CPU back, installed it, and the truck fired right up.
Over the course of all that, I got a pretty intimate look at all the logic and hardware from Toyota's official training manual.
The vernier-rheostat inside the throttle-position-sensor, and its variable resistance signal is a vital component of, and the CPU hasn't a clue what's happening without it's signal.
That may not be germane to your cars.